A COMPARISON OF NON TRADITIONAL INTERSECTION DESIGNS USING
MICROSCOPIC SIMULATION
By
Steve Chery
A Thesis Submitted to the Faculty of
College of Engineering and Computer Science
in Partial Fulfillment of the Requirements for the Degree of
Master of Science
Florida Atlantic University
Boca Raton, FL
May 2010
Copyright by Steve Chery 2010
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ACKNOWLEDGEMENTS
I would like to thank my advisors, Dr. Evangelos I. Kaisar and Jarice Rodriguez
Seda, for guidin and helping me throughout this process. Dr. Kaisar and Rodriguez’s knowledge and oversight were vital in all the progress made as well as the completion of this thesis. In addition, I would like to thank Dr. P. D. Scarlatos and Dr. Stevanovic for being part of my thesis committee. Their inputs and assistance were valuable in achieving the goals of the research. I would also like to acknowledge and thank Dr.
Praveen Edara from University of Missouri for his inputs in this research.
I want to thank all my family for their love and support at every step in my life. To my parents, especially, thank you for believing in me and keeping me in your prayers every day. Without you, nothing in my life would have been possible. I want to thank my friends at the Florida Atlantic University who helped me throughout this process, especially Scott Parr, Billy Degnan, Linda Hess, Alvaro and Borja Galletebeitia, Jean
Baptiste Edouard, Junior Senat, and everyone in the transportation group.
Finally, I want to extend my deepest appreciation to Rowolfia Jean Louis for her love and support throughout this experience. Her love, encouragement, and constant prayers made this research a success.
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ABSTRACT
Author: Steve Chery
Title: A Comparison of Non Traditional Intersection Designs Using
Microscopic Simulation
Institution: Florida Atlantic University
Thesis Advisors: Dr. Evangelos I. Kaisar and Jarice Rodriguez Seda
Degree: Master of Science
Year: 2010
In light of the growing traffic demand and the futility of the conventional solutions, many states have been considering alternative intersection designs. Researchers have demonstrated the benefits of several unconventional intersection designs and their implementation at different sites throughout the United States and abroad have delivered significant improvement in traffic compared to the conventional intersections. A signalized and un signalized roundabout, a Continuous Flow Intersection, and a Parallel
Flow Intersection have been evaluated and compared in this research as viable alternatives to the traditional single intersection. Using micro simulation platforms,
Aimsun 6.0 and VISSIM 5.10, models of each intersection are evaluated for low, medium, and high entrance volumes. The analysis revealed that the Roundabout performs better at low entering volumes while the Continuous flow yields better results at high volumes.
v DEDICATION
To my family and everyone who believed in me.
“Thank you”
A COMPARISON OF NON TRADITIONAL INTERSECTION DESIGNS USING
MICROSCOPIC SIMULATION
LIST OF TABLES ...... x
LIST OF FIGURES ...... xii
1 INTRODUCTION ...... 1
1.1 Motivation ...... 2
1.2 Problem Statement ...... 4
1.3 Overview of Approach ...... 7
2 LITERATURE REVIEW ...... 10
2.1 Un signalized Roundabout ...... 10
2.2 Signalized Roundabout ...... 24
2.3 Continuous Flow Intersection ...... 32
2.4 Parallel Flow Intersection ...... 43
2.5 Intersection Analysis – Analytical and Empirical Modeling...... 49
2.5.1 HCM Methodology for Unsignalized Intersections ...... 50
2.5.2 HCM Methodology for Signalized Intersections ...... 52
2.5.3 Roundabout Analysis ...... 54
2.6 Traffic Simulation Modeling ...... 55
2.7 Microsimulation versus Analytical/Empirical Modeling ...... 57
2.8 Comparison of Microsimulation Platforms ...... 58
2.9 Research Objectives ...... 61 vii
3 METHODOLOGY ...... 63
3.1 Geometric Elements ...... 64
3.2 Traffic Demand...... 67
3.3 Optimization Models ...... 69
4 SIMULATION STUDIES ...... 78
4.1 Aimsun 6.0 Simulation Platform ...... 78
4.2 VISSIM 5.10 Simulation Platform ...... 79
4.3 Calibration and Validation...... 80
4.4 Unsignalized Roundabout Network ...... 80
4.5 Signalized Roundabout Network ...... 82
4.6 Continuous Flow Intersection Network ...... 82
4.7 Parallel Flow Intersection Network ...... 83
4.8 Conventional Intersection Network ...... 84
4.9 Traffic Volume Data ...... 85
4.10 Signal Timing Plans ...... 89
4.11 Simulation Scenarios ...... 92
4.12 Measures of Effectiveness ...... 93
5 RESULTS AND ANALYSIS ...... 96
5.1 Balanced Traffic Scenarios ...... 97
5.2 Unbalanced Traffic Scenarios ...... 106
5.3 Analysis of Left Turn Movement ...... 109
5.4 Average Delay Time Sensitivity Analysis ...... 112
5.5 Comparison of AIMSUN and VISSIM ...... 116
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6 CONCLUSIONS ...... 120
6.1 Major Contributions of the Study ...... 123
6.2 Limitations of the Study ...... 124
6.3 Recommendations for Future Work ...... 124
7 REFERENCES ...... 126
8 APPENDIX ...... 132
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LIST OF TABLES
Table 2 1: Benefits of Implementing Signal Control to Conventional Roundabouts
(Hallworth, 1992) ...... 27
Table 2 2: Level of Service Criteria for Unsignalized Intersections ...... 52
Table 2 3: Level of Service Criteria for Signalized Intersections ...... 54
Table 3 1: Geometric Elements of the Modeled Double lane Roundabouts ...... 67
Table 3 2: Vehicular Flow Distribution Applied to Test Signal Optimization Model ..... 75
Table 4 1: Uniformly Balanced Flow with 20% Left Turn ...... 85
Table 4 2: Uniformly Balanced Flow with 20% Left Turn ...... 87
Table 4 3: Uniformly Balanced Flow with 25% Left Turn ...... 88
Table 4 4: Unbalanced Flow Condition Considering a Major and a Minor Road ...... 89
Table 4 5: Unbalanced Flow Condition Adopted from Bared and Edara (2005) ...... 89
Table 4 6: Optimized Signal Timing Plans for Conventional Intersection and Signalized
Roundabout for Balanced Flow Scenarios ...... 90
Table 4 7: Optimized Signal Timing Plans for Conventional Intersection and Signalized
Roundabout for Unbalanced Flow Scenarios ...... 91
Table 5 1: Average Delay Time Comparison for 15% Left Turn Movement for the
Balance Flow Scenario ...... 98
Table 5 2: Average Number of Stops Comparison for 15% Left Turn Movement for the
Balance Flow Scenario ...... 104
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Table 5 3: Average Delay Time Comparison for the Unbalanced Scenarios ...... 108
Table 5 4: Sensitivity Analysis Comparison for AIMSUN Results ...... 114
Table 5 5: Sensitivity Analysis Comparison for VISSIM Results ...... 115
Table 8 1: Average Delay Time Comparison for 20% Left Turn for the Balanced
Scenarios ...... 133
Table 8 2: Average Delay Time Comparison for 25% Left Turn for the Balanced
Scenarios ...... 134
Table 8 3: Average Number of Stops Comparison for 20% Left Turn for the Balanced
Scenarios ...... 135
Table 8 4: Average Number of Stops Comparison for 25% Left Turn for the Balanced
Scenarios ...... 136
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LIST OF FIGURES
Figure 2 1: Basic Geometric Elements of a Roundabout (US Department of
Transportation: Federal Highway Administration, 2000) ...... 12
Figure 2 2: Vehicular Movement at a Double Lane Roundabout (Center for
Transportation Research and Education: Iowa University, 2003) ...... 16
Figure 2 3: Comparison of Conflict Points between Conventional Intersection and a
Roundabout (US Department of Transportation: Federal Highway Administration,
2000)...... 17
Figure 2 4: Traffic Signal Options at a Roundabout (Stevens, 2005) ...... 25
Figure 2 5: Traffic Metering Options at a Signalized Roundabout (Stevens, 2005) ...... 31
Figure 2 6: Vehicular movement at a Continuous Flow Intersection (U.S. Department of Transportation: Federal Highway Administration, 2004) ...... 32
Figure 2 7: Signal Phasing at a Continuous Flow Intersection (U.S Department of
Transportation: Federal Highway Administration, 2004) ...... 34
Figure 2 8: Conflict Points at a Continuous Flow Intersection (FHWA) ...... 35
Figure 2 9: Comparison of Typical CFI and PFI Footprints and Turning Paths ...... 45
Figure 2 10: Vehicular Movement at a Parallel Flow Intersection (Quadrant
Engineering, 2009) ...... 46
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Figure 2 11: Comparison of Conflict Points between a Conventional Intersection and a Parallel Flow Intersection (Parsons, 2009) ...... 47
Figure 3 1: Methodology Flowchart ...... 64
Figure 3 2: Geometric Elements of Conventional Intersection AutoCAD Drawing ...... 66
Figure 3 3: Two lane Conventional Intersection Applied in the Case Study to Test the
Model ...... 73
Figure 3 4: Vehicular Movement at the Intersection (FHWA, 2004) ...... 74
Figure 3 5: Phase Grouping for the Case Study Intersection Following Federal
Highway Administration Guidelines (FHWA, 2004) ...... 74
Figure 3 6: Signal Timing Control Settings for the Continuous Flow Intersection
(Jagannathan and Bared, 2004) ...... 77
Figure 4 1: Screenshots of the Roundabout Models in AIMSUN (Left) and VISSIM
(Right) ...... 82
Figure 4 2: Screenshots of the Continuous Flow Intersection Models in AIMSUN
(Left) and VISSIM (Right) ...... 83
Figure 4 3: Screenshots of the Continuous Flow Intersection Models in AIMSUN
(Left) and VISSIM (Right) ...... 84
Figure 4 4: Screenshots of the Conventional Intersection Models in AIMSUN (Left) and VISSIM (Right) ...... 85
Figure 5 1: Average Delay Time Comparison for 15% Left Turn Movement for
Results Obtained from AIMSUN 6.0 ...... 102
Figure 5 2: Average Delay Time Comparison for 15% Left Turn Movement for
Results Obtained from VISSIM 5.10 ...... 102
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Figure 5 3: Average Number of Stops Comparison for 15% Left Turn Movement for
Results Obtained from AIMSUN 6.0 ...... 105
Figure 5 4: Average Number of Stops Comparison for 15% Left Turn Movement for
Results Obtained from VISSIM 5.10 ...... 106
Figure 5 5: Average Delay Time Comparison for Roundabout for Different Left Turn
Movement...... 109
Figure 5 6: Average Delay Time Comparison for Parallel Flow Intersection for
Different Left Turn Movement ...... 111
Figure 5 7: Average Delay Time Comparison for Continuous Flow Intersection for
Different Left Turn Movement ...... 111
Figure 5 8: AIMSUN vs. VISSIM Average Delay Time for Continuous Flow
Intersection for 15% Left Turn Movement ...... 116
Figure 8 1: Geometric Elements of Parallel Flow Intersection...... 132
Figure 8 2: Geometric Elements of Continuous Flow Intersection ...... 132
Figure 8 3: Average Delay Time Comparison for 25% Left Turn Movement for
Results Obtained from AIMSUN 6.0 ...... 137
Figure 8 4: Average Number of Stops Comparison for 25% Left Turn Movement for
Results Obtained from AIMSUN 6.0 ...... 137
Figure 8 5: Average Delay Time Comparison for 25% Left Turn Movement for
Results Obtained from VISSIM 5.10 ...... 138
Figure 8 6: Average Number of Stops Comparison for 25% Left Turn Movement for
Results Obtained from VISSIM 5.10 ...... 138
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Figure 8 7: Average Delay Time Comparison for Parallel Flow Intersection for
Different Left Turn Movement ...... 139
Figure 8 8: Average Number of Stops Comparison for Conventional Intersection for
Different Left Turn Movement ...... 139
Figure 8 9: Average Number of Stops Comparison for Roundabout for Different Left
Turn Movement ...... 140
Figure 8 10: Average Number of Stops Comparison for Continuous Flow Intersection for Different Left Turn Movement ...... 140
Figure 8 11: Average Number of Stops Comparison for Parallel Flow Intersection for Different Left Turn Movement ...... 141
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1 INTRODUCTION
In an industrialized country such as the United States, mobility is considered a necessity. The effectiveness of the transportation system is a vital constituent of people’s daily lives as they commute between different points of interest whether it is work, school, church, or their favorite shopping centers. The surface transportation system can be regarded as a network embodied as a set of links and nodes. The links in the road network represent arterial roads including freeways, major highways and local streets, interconnected by means of interchanges and intersections representing the nodes.
In their work, measuring the structure of road networks, Xie et al., (2006) details the functional and operational classifications of the links in the road network. With local streets and arterial roads respectively emphasizing a land access function and a high level of mobility for through movement, roads are designed to serve different purposes, which can be referred to as “functional classes.” The operational classification of the roads is further explained in terms of their levels of service, incorporating elements such as riding comfort and freedom from speed changes (Xie et al., 2006).
The quality of the transportation networks, however, does not solely depend on the characteristics of the links, but also on the way the links are connected (Ritveld, 1995).
As one travels on a road network consisting of different classes of links, transferring from one class of roads to another is essential. Therein lays the substance and the need for efficient intersections and interchanges in that they provide a way of transfer to travelers
1
on the road network changing from one class of road to another in their search of the most efficient route.
Although alternative modes are generally available, for instance public transportation, personal vehicles offer the benefits of freedom and privacy. Commuters especially in places where the points of interest are decentralized therefore prefer the latter, which, as a result, creates an ever increasing traffic demand on the road network.
The influx of traffic, especially during peak hours of the day when more trips are generated as people travel to and from their points of interest, renders the road network inefficient. In American cities, junctions with three and four connections usually referred to as “T junctions” and “X junctions” respectively, are the two dominant intersection patterns (Marshall, 2005). These traditional intersections play an important role in the transportation network especially in urban and downtowns areas where intersections are closely spaced. However, the conventional intersections often operate at or over capacity due to increase in traffic demand. Drivers, thus, experience longer delays and more frequent stops, the effects of which may be detrimental in terms of lost time, accidents, and environmentally harmful emission.
1.1 Motivation
As population continues to grow in the urban and suburban areas, the congestion level on the road networks has also exacerbated. Anthony Downs, in a report titled “Why
Traffic Congestion Is Here to stay…and Will Get Worse,” explains that traffic congestion in the US is certain to get worse in the next few decades mainly because of rising populations and wealth. This outcome, however, should not be viewed as a mark of
2
social failure or wrong policies because congestion reflects economic prosperity (Downs,
2004).
Meanwhile, traffic congestion is costing US citizens billions of dollars year round.
In 2002, it was estimated that traffic congestion costs the United States more than $67 billion annually while approximately 6 billion gallons of fuel, and 3.6 billion of hours are wasted idling in traffic jams. For the average person, this means $1,160 and 62 hours wasted annually by congestion (Kennedy, 2003). According to the Texas Transportation
Institute’s “2009 Urban Mobility Report,” in 2007, the cost of congestion to Americans was 4.2 billion hours of lost time and more importantly, $87.2 billion, an increase of over
20 billion in 5 years and “more than 50% over the previous decade (Lomax et al., 2009).
A single busy conventional intersection cost the public $26,243 per workday, which is equivalent to $6.6 million per year (Parsons, 2009).
Furthermore, accidents are another factor, which greatly affects the performance of the road networks. Over the past decades, U.S intersection designs have improved to incorporate more safety measures. For instance, the Florida Intersection Design Guide delineates certain requirements for all new construction and major reconstruction of at grade intersections on the state highway system (Florida Department of Transportation,
2007). Following the standards as described in the guide, an intersection must be designed so that it provides:
Safe and convenient operation for all road users, including cyclists and
pedestrians;
Adequate maneuvering space for design vehicles;
Resolution of conflicts between competing movements;
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Adequate visibility of conflicting traffic;
Suitable advance warnings of all hazards etc
Yet, the annual number of intersection related accidents has not significantly improved. In 2004, with more than 2.7 million crashes, intersection related accidents account for over 45 percent of all crashes in the United States. In addition, approximately 900,000 crashes, which account for nearly 45 percent of all injury crashes, occurred at intersections. Overall, that year alone, intersection crashes amount to a financial loss of $96 billion (US Department of Transportation: Federal Highway
Administration, 2006).
1.2 Problem Statement
Traffic congestion is not expected to disappear in the near future. However, to counter future growth in traffic demand, innovative measures are needed to alleviate the situation. Improvements in traffic signal control strategies overtime have made signal timing optimization usually the first and favored alternative to traffic agencies. Before, traffic signal setting was based on fixed time strategies, which use historical traffic data and propose a signal plan for a particular time of the day. In that case, the traffic optimization problem is solved offline and the timing plan is implemented when needed.
Nevertheless, traffic signal setting can now be based on traffic responsive or real time strategies where real time data are used to determine timings for immediate implementation (Papageorgiou, 2003). Signal timing optimization is convenient in terms that the new timing plans can be determined and implemented with any variation of traffic flows and cost efficient in that no construction is required.
4
In many cases where traffic demand far exceeds the capacity of the intersections, however, traffic signal control strategies can be inefficient. Lane addition is another resort available to traffic engineers and has been used extensively to increase the capacity of the traditional intersection. Although adding turning or through lanes may provide a short term relief, that solution is often unfeasible because of spatial constraints or it may simply be too expensive to widen the existing intersection.
In light of the growing traffic demand and the futility of the conventional solutions, many states have been considering alternative intersection designs. Researchers have demonstrated the benefits of several unconventional intersection designs and their implementation at different sites throughout the United States and abroad have delivered significant improvement in traffic compared to the conventional intersections.
Developed in the United Kingdom in the 1960s to address issues of safety and efficiency related to early uses of traffic circles in the United States in the early 20 th century, roundabouts have been shown to be a safe and effective alternative to designers and traffic engineers considering improvements to a conventional intersection (National
Cooperative Research Highway Program, 2007). Roundabouts have since been widely used internationally. In the United States, the Federal Highway Administration recommends the use of roundabouts as a way to improve intersection performance citing benefits including less severe crashes than other intersection crashes; increased traffic capacity and improved traffic flow; and aesthetics values with a deployment goal of building approximately 1,000 roundabouts per year (US Department of Transportation:
Federal Highway Administration, 2006).
5
Other types of innovative intersection designs have also been shown to improve both operational and safety performance issues associated with conventional intersections.
The benefits provided by these novel designs are attained mostly by rerouting and prohibiting left turning movement at the main junction of the intersection. Some examples of this type of design include the median u turn, the bowtie, continuous flow.
The median u turn crossover eliminates left turns at intersections and move them to median crossovers located beyond the main intersection (US Department of
Transportation: Federal Highway Administration, 2004). The bowtie intersection uses two roundabouts on the minor cross street and forces vehicles turning left to make a u turn at the roundabouts instead at the main intersection (Reid and Hummer, 2001). The continuous flow intersection eliminates left turn conflicts with opposing through movement by displacing the left turn lane to the left side of the road at a distance upstream of the main intersection. Many researchers have studied and compared a variation of these designs. Although each individual intersection offers particular strength and weaknesses, the continuous flow intersection has been shown to produce results that, not only surpass other designs but also comparable to grade separation.
Recently developed and published by Florida based engineer Greg Parsons, the parallel flow intersection reroutes left turns as well (Quadrant Engineering, 2009). Past studies have demonstrated that the parallel flow intersection yields results that considerably improve traffic from conventional intersections and at the same time, are similar to that of the continuous flow intersection (Parsons, 2007; Cheong et al., 2008). Although conventional intersections are an important element of the surface transportation network, oftentimes they are unable to accommodate heavy traffic flow and heavy left turn
6
movement, which, in turn may result in high delays and accidents. Hence, the use of unconventional intersection designs have become an essential tool to reduce congestion and improve safety.
The main objective of this research is to evaluate the operational performance of several unconventional intersection schemes, mainly both an un signalized and signalized roundabout, a continuous flow intersection, and parallel flow intersection. Each unconventional intersection design is to be thoroughly analyzed and their performance results compared to each other as well as a comparable conventional intersection in order to specify an appropriate unconventional intersection design for different problem locations. The specific goals to be completed to achieve the objective of this research are as follows:
Develop a realistic microscopic simulation model of each intersection
scheme
Validate and calibrate the microscopic models
Develop traffic demand data that represent conditions encountered in
the field
Study and quantify the effect of left turn movement for each
intersection design
1.3 Overview of Approach
To evaluate the operational performance of the intersection designs being compared in this research, two microscopic simulation platforms: Aimsun 6.0 developed and marketed by TSS – Transport Simulation System based in Barcelona, Spain and VISSIM
5.10 from PTV – Planung Transport Verkehr AG of Germany, were selected. The
7
performance measures used were average delay time and number of stops. According to the Highway Capacity Manual, delay is the primary measure of effectiveness for both signalized and unsignalized intersections. As a performance measure, number of stops clarifies the number of time, on average, a driver has to stop while travelling through each type of intersection. These measures of effectiveness are selected because they illustrate driver discomfort on the road.
Hypothetical volumes required for the analysis of the intersection schemes were generated with the consideration of three critical issues. First, each intersection was evaluated under different volume levels ranging from low to high in order to simulate both peak and off peak conditions. The second issue considered was testing the intersection designs under balanced and unbalanced flow condition. Under a balanced flow condition, the volumes on all four approaches are similar whereas an unbalanced volume condition considers a major and minor road at the intersection. The third issue was to quantify the effect of increasing left turn volume on the performance of the intersection. Three different scenarios with 15%, 20%, and 25% left turn volume were modeled for the balanced flow scenarios to fulfill that issue.
The following chapters of the study describe the overall approach to the problem:
• A literature review is performed in order to ascertain the characteristics
of each intersection design as well as summarize some of the previous works
aimed at comparing the particular intersections being considered for
comparison in this research
• The methodology chapter gives an overview of the necessary steps
followed to solve the problem describing the geometries of the intersections,
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the traffic demand data required for the analysis, and the optimization procedures used to obtain optimum signal setting for each of the intersections.
• The simulation studies chapter describes the simulation process and the procedure used to develop models of each intersection scheme. The basis for the selection of the microscopic simulation platforms namely AIMSUN and
VISSIM are discussed. The performance measures used this study as well as measures of effectiveness from previous works are also presented
• The results and analysis chapter shows how each intersection performs under the selected scenarios and allows the reader to see the comparison between each intersection scheme
• Finally, the conclusions specify where the use of each intersection scheme is appropriate. In addition, the conclusions section points out the limitations of the study and recommendations for future work
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2 LITERATURE REVIEW
The primary goal of this research is to evaluate and compare the operational performance of several non traditional intersections including a Roundabout, a Signalized
Roundabout, a Continuous Flow Intersection, and a Parallel Flow Intersection, using simulation platforms. The results of this comparison will assist decision makers in selecting a suitable alternative to the typical four legged intersection in cases where the latter fails to perform safely and efficiently ensuing excess congestion and potential incidents. A thorough literature review, focusing on the characteristics and potential benefits and liabilities of each intersection, the procedures for analyzing intersections, different types of simulation explicitly macroscopic, mesoscopic, and microscopic simulation, a review of available simulation platforms, as well as results from previous studies, was performed in order to attain the goal of this research.
2.1 Un signalized Roundabout
The Federal Highway Administration’s roundabout guide defines roundabouts as
“circular intersections with specific design and traffic control features which include yield control of all entering traffic, channelized approaches, and appropriate geometric curvature to ensure that travel speeds on the circulatory roadway are typically less than
50 km/h (30 mph)” (US Department of Transportation: Federal Highway Administration,
2000). The main geometrical components of a roundabout, as illustrated in figure 2 1,
10
consist of a central island, a splitter island, circulatory roadway, apron, yield lines, accessible pedestrian crossings, bicycle treatments, and landscaping buffers. These components are key elements that help stipulate whether a circulatory roadway can be classified as a roundabout. The purpose of each of the components, as defined in the
FHWA’s “Roundabouts: an Informational Guide (2000),” is as follows:
• Central island: the raised area in the center of a roundabout around which
traffic circulates.
• Splitter island A splitter island is a raised or painted area on an approach
used to separate entering from exiting traffic, deflect and slow entering traffic,
and provide storage space for pedestrians crossing the road in two stages.
• Circulatory roadway: the curved path used by vehicles to travel in a
counterclockwise fashion around the central island
• Apron: the mountable portion of the central island adjacent to the
circulatory roadway used if required on smaller roundabouts to accommodate
the wheel tracking of large vehicles
• Yield line: a pavement marking used to mark the point of entry from an
approach into the circulatory roadway. It is generally marked along the
inscribed circle. Entering vehicles must yield to any circulating traffic coming
from the left before crossing this line into the circulatory roadway.
• Accessible pedestrian crossings: should be provided at all roundabouts. The
crossing location is set back from the yield line, and the splitter island is cut to
allow pedestrians, wheelchairs, strollers, and bicycles to pass through.
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• Bicycle treatments: provide bicyclists the option of traveling through the
roundabout either as a vehicle or as a pedestrian, depending on the bicyclist’s
level of comfort.
• Landscaping buffer: are provided at most roundabouts to separate vehicular
and pedestrian traffic and to encourage pedestrians to cross only at the
designated crossing locations. Landscaping buffers can also significantly
improve the aesthetics of the intersection
Figure 2 1: Basic Geometric Elements of a Roundabout (US Department of Transportation: Federal
Highway Administration, 2000)
Roundabouts have been used successfully throughout the world and are increasingly being used in the United States since 1990 (Transportation Research Board: Highway
12
Capacity Manual, 2000). According to the Federal Highway Administration’s
“Roundabouts: An Informational Guide,” designing the geometry of roundabouts involves choosing between trade offs of safety and capacity. The geometric characteristics of a roundabout such as the horizontal curvature and narrow pavement widths are used to produce a reduced speed environment, forcing traffic to enter and circulate inside the intersection at low speed and therefore operating safer than a conventional intersection (Robinson et al., 2000). However, as the width of entry and radii are reduced, the capacity of the roundabout is also reduced. Hence, designing a roundabout involves determining the optimal balance between safety and operational performance (Robinson et. al., 2000).
The design and the way a roundabout operates is dominated by different factors. For instance, although certain basic fundamentals such as determining the inscribed circle diameter, splitter island, entry and exit width, adequate intersection and sopping sight distance, bicycle and pedestrian provision etc. apply to both single lane and double lane roundabouts, designing the geometries of the double lane roundabout is more complicated. Special considerations should be made for the fact that multiple traffic streams are able to enter, circulate, and exit the roundabout side by side (US Department of Transportation: Federal Highway Administration, 2000).
Guidelines specifying the allowable movements within double lane roundabouts so that vehicle paths do not cross have not been widely adopted in the United States until recently. As roundabouts started gaining popularity in the U.S. during the early 2000’s, a clear guideline, providing motorist with adequate information on the way traffic moves inside double lane roundabouts, was non existent. Within the roundabout design
13
community, there was no consensus about whether specific guidance is necessary, a fact, which is substantiated by the variation among roundabout, designs throughout the country (Kinzel, 2003). Roundabout designers followed one of two distinct approaches:
• The “laissez faire” approach, in which no markings within the roundabout
and no advance lane usage signage are provided. As this approach does not
discourage lane changes within the roundabout, the circulatory roadway
functions as a wide area in which motorists can and often must fight for
position. Some motorists may not be comfortable entering the roundabout side
by side under this scheme due to the uncertainty of their neighbors’ intention
while circulating within the roundabout, which in turn, creates potential for
more accidents.
• The “positive guidance” approach reinforces lane discipline within the
circulatory roadway, typically via circulatory striping and advance lane use
control signs. Circulatory striping designs often attempt to match the
circulating lane choice with the exit lane choice. This approach instills more
comfort in motorists entering the roundabout side by side, as lane positioning is
clearly specified (Kinzel, 2003).
The FHWA’s guide to roundabout design does not validate the need for lane usage signage at multi lane roundabouts. On the contrary, the “laissez faire” approach is authenticated throughout the guide on several occasions. The guide states, “There is no international consensus of the effectiveness of lane use signs and/or pavement markings
[at multi lane roundabout]” (US Department of Transportation: Federal Highway
Administration, 2000). At the time, roundabouts were just gaining popularity and
14
acceptance within the United States and drivers were generally unfamiliar with the circulation rules of roundabouts. Consequently, the FHWA roundabout guide
“recommended that double lane roundabouts be designed to avoid the use of lane control signs wherever possible, at least until drivers become more accustomed to driving in roundabouts” (US Department of Transportation: Federal Highway Administration,
2000).
A consensus on how traffic circulates within a roundabout has since been reached and many state as well as the federal departments of transportation in the United States have adopted the vehicular movement as exemplified in figure 2 2. A driver, upon entering a multi lane roundabout, will see two signs as he/she approaches the intersection: The yellow "roundabout ahead" sign and a black and white "lane choice" sign. The driver must choose a lane prior to entering the roundabout in the same manner that lanes are selected in a traditional multi lane intersection. A driver choosing the right lane of the roundabout has the ability to go through or make a right turn. Choosing the left lane will give the driver the choice to go through, turn left, as well as perform a U turn around the roundabout. At the dashed yield line at the entrance of the roundabout, the driver must yield and priority is given to drivers already circulating in the roundabout
(Washington State Department of Transportation, 2008).
The microscopic simulation models of the double lane roundabout being evaluated in this research encompass the necessary components of a roundabout as previously described. The guideline specifying the allowable vehicular movement at double lane roundabouts are followed and the above procedure has been carefully implemented in the simulation models of the double lane roundabout.
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Figure 2 2: Vehicular Movement at a Double Lane Roundabout (Center for Transportation
Research and Education: Iowa University, 2003)
Previous researchers have evaluated the benefits and disadvantages of roundabout compared to a conventional four legged intersection. A single lane roundabout reduces the number of conflict points at a conventional intersection design from thirty two to eight. As shown in Figure 2 3, a conventional intersection encompasses eight diverging, eight merging, and sixteen crossing conflict points while the single lane roundabout has only four diverging and four merging conflict points. Adding a circulatory lane within the roundabout increases the total conflict points at a double lane roundabout to sixteen that still has 50% less conflict points compared to a conventional intersection. 16
Figure 2 3: Comparison of Conflict Points between Conventional Intersection and a Roundabout (US
Department of Transportation: Federal Highway Administration, 2000)
The operational and safety performance of roundabouts have been assessed and compared to that of conventional intersections by various researchers and transportation engineers considering unconventional designs to solve the issues typically encountered at many conventional intersections in the field. However, in the United States, “there is not sufficient literature available for comparing the performance of roundabouts with those of signalized intersections from low to high volume conditions” (Mishra, 2009).
Thorson et al. (2001) from the Research Division of the Nevada Department of
Transportation compared a roundabout operation to four way stop and signalized intersections. Prior to the installation of the roundabout at that particular location, the intersection was controlled by a four way stop. At relatively high peak volumes, commuters at the intersection experienced lengthy delays on two approaches. The purpose of the study was to demonstrate to decision makers as well as the public the benefits of converting the four way stop intersection to a roundabout. Signalization of
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the intersection was also considered as an alternative. The microscopic simulation platform NETSIM (Network Simulation), a part of the software package TSIS (Traffic
Software Integrated System) developed by ITT Systems and Science Corporation with the support of the Federal Highway Administration, was used to model the three alternatives. Data was collected from the field at the time of the study. The afternoon peak hour traffic volumes used in the analysis of the intersections was: 680 vehicles per hour (vph) in the northbound direction, 740 vph southbound, 400 vph eastbound, and 160 vph on the westbound direction. The run time for each simulation was thirty minutes.
Average delay time and fuel consumption for the intersections were used as measures of effectiveness generated from the NETSIM simulation platform. The results show that the roundabout outperformed both the four way stop and the signalized intersection. The four way stop and the signalized intersections produced average delays of 100 and approximately 50 seconds per vehicle respectively, while the roundabout had an average delay of approximately 10 seconds per vehicle.
Mark T. Johnson of the Wisconsin Department of Transportation and William Hange from the City of Loveland, Colorado (2002) examined the benefits of roundabouts as an alternative to traffic signals under various conditions. The study aimed to provide traffic engineers the understanding of how the differences in roundabout and traffic signal operation can be used to achieve safe and efficient traffic operation, reduce congestion, and lessen the negative impacts often associated with roadway and highway projects. To attain this goal, several example projects, encompassing both developing areas and existing infrastructures that needed capacity and safety improvements, were selected to illustrate that the operational characteristics of roundabouts better achieved the particular
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project objectives than the signalized counterparts. Five case studies were chosen from different states. The first case study project is located in Loveland, Colorado where two double lane roundabouts were constructed in August of 1998 as part of a large mixed used development. One of the roundabouts was a new construction and the other replaced an existing conventional intersection controlled by traffic signals. The roundabouts served an existing average daily traffic (ADT) of 20,000 and have peak hour design volumes of 4,500 vehicles. Implementing the roundabouts at that particular site provided several major benefits including:
• Full access to adjacent properties which was previously denied
• Improved site circulation
• Improved safety (five property damage only incidents, zero injury, zero
pedestrian or bicycle crashes since 1998).
Another case study project presented in the study was a proposed improvement to an existing conventional signalized intersection in Watertown, Wisconsin. The proposed improvement to the intersection includes widening the roadway from a two lane to a four lane in order to provide congestion relief with the expected increase in traffic demand in that area. The design hour volume at the intersection was 3,200 vehicles with a split of 60/40 favoring the major mainline. Trucks represented ten percent of the total volume. While widening the roadway to a four lane offered good congestion relief for the major road at the signalized intersection, only a marginal capacity improvement for the large number of left turning vehicles accessing the businesses located off the side streets was provided. A double lane roundabout was studied as an alternative to the proposed intersection widening project. The researchers demonstrated that the
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characteristics of the roundabout alternative provided an excellent level of service (LOS
A) under existing conditions and future volumes (LOS C). Access to existing businesses as well as pedestrian flow was also improved. It was concluded that, for this particular location, the roundabout alternative could use the space available near the intersection, provide excellent operations, and meet the project objectives.
Several other projects were studied to determine whether roundabouts might be a potential alternative to the signal controlled conventional intersections. The major benefits gained from the roundabouts in most of these cases include:
• Better overall traffic operations
• Reduced residential impacts
• Excellent pedestrian/bicycle connectivity
• Improved safety
• Aesthetics gateway opportunity and
• Enhanced livability
Overall, Johnson and Hange (2002) found that roundabouts are suitable for low, medium, and high volume situations. While there may be situations when signals will
“simply outperform roundabouts,” signals and roundabouts are not mutually exclusive.
They are in fact compatible and can be used as a ‘systems approach’ in order to achieve optimal operations (Johnson and Hange, 2002).
As traffic engineers and researchers seek out innovations that enhance traffic flow, ultimately leading to alleviation of congestion, Bared and Kaisar (2002) investigated the roundabout as an alternative to diamond interchanges. In the study, the authors compared the delay caused by a diamond interchange with the delays at interchanges containing
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double or single roundabouts. Data generated from computer simulated models for roundabout traffic operations and for signalized intersections were used as the basis for the comparisons.
The three interchange configurations selected for the analysis: the diamond, the double roundabout, and the single roundabout, had comparable geometries. For the diamond interchange, the crossroad had four through lanes: two in each direction with exclusive 76 meter right turn and 106 meter left turn lanes. The two intersections were offset by 90 meters from stop bar to stop bar. The off ramps for the diamond interchange were flared from one lane to two lanes at the entry to provide a 60 meter right turn lane.
Similarly, the double roundabout and single roundabout interchange off ramps were flared from one lane to two lanes at the entry from 5 meters to 9 meters total width.
Texas Transportation Institute’s PASSER III software was used to determine the cycle length (ranging from 60 to 120 seconds), optimum phase timing, and time offset between the two signals for the interchanges. The Federal Highway Administration’s traffic microsimulation model CORSIM was used to model the interchanges and estimate stop delay.
The following scenarios were considered in the study:
• Weekday peak 30 percent left turning traffic volume from the cross
street and 60 percent left turning volume from the off ramps
• Weekday off peak 20 percent left turning volume from the cross street
and 40 percent left turning volume from the off ramps
• Weekend 20 percent left turning volume from the cross street and 60
percent left turning volume from the off ramps
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The results of the study showed that traffic operation is more efficient in roundabouts than at diamond interchanges for low to moderate traffic flows of up to total entering volumes of around 4,500 vehicles per hour. Using double roundabouts at interchange ramp terminals with low and medium flows resulted in noticeably less delay than stop controlled and signalized diamond interchanges. Moreover, the delay benefits for single roundabout interchanges in tight urban settings were similarly significant compared to conventional diamond interchanges.
The work of Bared and Edara (2005) scrutinized the simulated capacity of roundabouts and the impact of roundabouts within a progressed signalized road. Traffic simulation was used to study the performance of urban single lane and double lane roundabouts in isolation. The impact of signalized intersection proximity to roundabouts was also studied by inserting a roundabout within an arterial corridor. VISSIM traffic simulation software was used to model the roundabouts. The models were created by importing a CAD layout of the roundabouts into the VISSIM software and set as a background on which the VISSIM links were drawn. The number of lanes, lane widths, gradients, and speeds on the approaches were set to represent the desired conditions.
Speeds on the approaches were set between 30 miles per hour (mph) to 36 mph for cars and 25 mph to 28 mph for trucks. On the entries, circulating, and exiting curves speeds of 15 mph to 18 mph and 12 mph to 15 mph were set for cars and trucks respectively.
Priority rules, which check for minimum gap time and minimum headway, were set for entering and exiting movements at the roundabouts. Following the priority rules in the
VISSIM model, vehicles enter the roundabout solely when the time gap and headway as measured from the conflict marker are greater than the respective minimum values,
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which are set partly based on field experience and partly on observing the simulation animation to have no visible collision between vehicles. Capacities for the modeled roundabouts were determined by flooding an entry at a time and facilitating a wide variety of circulating volumes. The maximum entry capacity and the conflicting flow for each approach were determined from the VISSIM simulation assuming that capacity for the approach is reached when the throughput is less than the input volume by more than
100 vph and average delay for that approach exceeds 70 seconds. Repeating the same procedure for different traffic scenarios, the maximum entry capacity vs. conflicting flow was obtained for the single lane and double lane roundabout models.
The double lane roundabout model was then inserted in a section of an arterial consisting of three signalized intersections separated by a ¼ mile each in order to study the operational impact of signalized intersection proximity to roundabouts. The arterial consisted of two through lanes and one exclusive left turn and right turn lanes. This procedure was performed in order to test the hypothesis that replacing a signalized intersection within an arterial with a roundabout would not worsen the overall traffic performance. Both alternatives were simulated in VISSIM for three hypothetical traffic flow scenarios representing low, medium, and high volumes. The signals were coordinated and had a 60 seconds cycle length. Signal optimization software,
TRANSYT 7F was used to obtain the signal coordination. Average delays per vehicle for the networks were recorded from the simulation in order to compare the two alternatives. The simulation results showed that the roundabout performs better than the signalized intersection when operating at or below capacity. However, when the
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roundabout approaches are operating near capacity, the fully signalized network produced slightly lower overall delay.
According to Johnson and Hange (2002), roundabouts are not a ‘panacea’. They require the application of thorough traffic engineering principles and sound design techniques in order to achieve proper operations (Johnson and Hange, 2002). The studies presented above indicate that when roundabouts are designed following the proper engineering guidelines, they provide benefits that far outweigh the conventional intersection designs. Roundabouts can be used to replace conventional intersection designs for low, medium, and high volume situations and their operational performance have been proven to surpass that of comparable conventional designs in most cases.
2.2 Signalized Roundabout
The use of the modern roundabout as an effective form of traffic control in the
United States has been increasing in the last decade. However, more and more information is needed when situations arise in which additional traffic control is required.
The signalization and metering of roundabouts can relieve congestion during peak hours of the day in addition to possibly providing safer access for pedestrians and cyclists
(Robinson et al., 2000).
Most of the published literature related to signalized roundabouts is based on experience in the United Kingdom where signals were first adopted in order to control traffic flow at many of the large roundabouts. Hallworth (1992) classified several different options for implementing signal control to a roundabout. Depending on the size of the roundabout, the location, and the traffic conditions, signalization for a particular
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site can be a combination of different options. The flow chart in figure 1 presents these options as identified by Hallworth.
Figure 2 4: Traffic Signal Options at a Roundabout (Stevens, 2005)
The means of control at a signalized roundabout describes how the signal system controls entering and exiting vehicles. There are two main means of control at a signalized roundabout: direct control where both external and internal approaches are affected, influencing traffic entering the roundabout as well as vehicles leaving from within the roundabout; and indirect control where only external traffic at a distance from the entry point of the roundabout. The circulatory traffic within the roundabout is not affected. Indirect control of vehicles is sometimes established with the addition of pedestrian signals, where crosswalks are at a distance from the roundabout entry
(Hallworth 1992).
The time of operation at a signalized roundabout focuses on the period of time a signal operates. Two times of operation are commonly used at signalized roundabouts: full time and part time. For full time operation, the installed signals operate permanently and do not turn off during non peak times. For part time operation, the installed signal does not operate at all times. The signal is activated by time of day or by detectors.
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Detectors are usually placed at a distance from the controlled approach on a delay setting to determine when a queue has built up (Hallworth, 1992).
Approach control describes the number of approaches controlled with a signal. There are two main types of approach control: full control and part control. Full approach control regulates all approaches of the roundabout. Part approach control at a signalized roundabout regulates one or more, but not all, legs of a roundabout while remaining approaches operate under right of way control (Hallworth, 1992).
Although the intended purpose of a roundabout is to eliminate the need for signals at an intersection, the implementation of signal control at a roundabout can be quite useful.
Hallworth (1992) also noted the benefits of introducing signal control to roundabouts on all approaches. These benefits are summarized in the following table:
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Table 2 1: Benefits of Implementing Signal Control to Conventional Roundabouts (Hallworth, 1992)
Compared MOEs Roundabout Signalized Roundabout Delays Delays on some Signals can be used to approaches can become alter the natural priority excessive due to to provide more balanced unbalanced flows delays Queues Queues on particular Signals can monitor the approaches can exceed a queue lengths and bias critical length the green times so as to reduce the critical queue Capacity The overall roundabout Signals can improve the capacity may prove overall capacity insufficient Safety / Control The need for weaving and Signals can better merging can provide regulate traffic patterns, difficulties at particular reduce the need for entry approaches weaving and merging and reduce speeds Pedestrian Facilities Lack of control can make Signals can render it safer it difficult for pedestrians and more positive to cross approaches
Many traffic agencies throughout the world have been experimenting with several types of traffic signal controls at roundabouts to the ends of improving vehicular and pedestrian safety, reduce delays, and increase capacity. According to Akcelik (2005), metering signals, for instance, have been used at roundabouts in Australia, United
Kingdom, and the United States to alleviate the problem of excessive delay and queuing by creating gaps in the circulating stream.
Bernetti et al., (2003) compares performances of un signalized and signalized roundabouts under critical traffic conditions in Italy. Capacity and delays were the two performance measures used as the basis of the comparison between the two alternative intersection designs. Two different roundabouts were selected in the study: a multi lane signalized roundabout located in Milan, which serves approximately 7,000 vehicles per
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hour and a proposed unsignalized double lane roundabout near Venice with a traffic volume of 3,000 vehicles per hour.
To evaluate the performance of the unsignalized roundabout, two methods were tested and compared: the empirical approach proposed by SETRA from France and the
Australian approach, which is based on gap acceptance. For the signalized cases, a mesoscopic model for evaluating performance of signalized intersections proposed by
Bernetti, Camus, and Longo (2002) was used.
The results of this comparison showed that at low traffic volumes, queuing delays for the unsignalized roundabout were ‘negligible’ and the overall performance of the unsignalized cases was better than that of the signalized cases. However, at high traffic volumes, the signalized roundabouts produced balanced delays to vehicles of all approaching legs and improved the overall capacity of the roundabout. Moreover, at high traffic flows, especially in the cases where volumes were predominant in one specific direction, the unsignalized roundabout was unable to self regulate while the signalized roundabout better managed the influence of unbalanced flows.
Akcelik has investigated the performance of metering signals and has proposed a method for the analysis of capacity and performance of roundabouts operating with metering signals. The author presented different cases of various roundabouts where metering signals were used or considered for use including various locations in
Melbourne, Australia and Clearwater, Florida in the United States where an unbalanced flow condition creates heavy delays at some of the roundabout entries.
A roundabout with metering signals in Melbourne, Australia (Mickleham Rd and
Broadmeadows Rd) was used as a case study in this analysis. Because of unbalanced
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flow condition at that particular site, heavy traffic movement from Mickleham Rd South to Broadmeadows Rd caused long delays to the other heavy traffic Mickleham Rd North to south, with extensive queuing of 500 meters to 600 meters occurring regularly during the morning peak on the Mickleham Rd north approach.
The effects of metering signals were modeled using aaSIDRA, an empirical platform developed by Akcelik & Associates to model roundabout capacity and performance, by applying the following analysis method, which involved estimating operating characteristics for three operation scenarios:
• Base conditions represent the roundabout operating with blank metering
signals on the metered approach, which corresponds, to normal operation of
the roundabout without metering signals.
• Roundabout operation when the metering signals display RED, which
means that the metered approach traffic is stopped and the rest of the
roundabout operates according to normal roundabout rules
• Signalized intersection scenario to reproduce the operation of metered
approach signals in order to determine the performance of the metered
approach
The results of the analysis indicated that the metering signals significantly reduced delay and queue length on the controlling approach by 20 to 40 percent but increase the delay and queue length on the metered approach.
In the United States, a lack of formal experience among professionals in roundabout signalization and metering propelled Stevens (2005) to develop guidelines for implementing signals and meters at roundabouts using information from survey responses
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and literature. The guidelines present several options for the signalization and metering of roundabouts including options on means of control, time of operation, and approach control. Around the timeline of Stevens’ research, there had been few reported cases of roundabout signalization in the United States including pedestrian actuated traffic signals installed in Florida and Utah, and a metering device being considered to help ease congestion during certain periods of peak traffic flow in Maryland (Sides, 2000). The primary objectives of Stevens’ research were as follows:
• Obtain general information and methods on the signalization and metering of
roundabouts
• Determine locations and features of roundabouts that have traffic signals and
meters in the United States, Europe, and Australia
• Determine the reasons for signalization and metering
• Determine locations where signalization and metering have been planned or
could help operations
• Highlight the results of traffic signalization and metering and discuss their
effectiveness
• Develop a set of guidelines for traffic control signals and meters at a roundabout
• Gain feedback on the developed set of guidelines from known roundabout
experts in roundabout design and operations relative to the applicability of the
guidelines
• Finalize a set of guidelines applicable to the signalization and metering of
roundabouts
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To attain his research objectives and finalize a set of guidelines, a literature review of documents related to the signalization and metering of roundabouts was conducted using resources from library and internet. In addition, a web survey was created to obtain information from known experts in the field of roundabout design and operation. Based on the literature and the surveys received, a set of guidelines was established. It should be noted that these guidelines were not developed on technical standards rather from recurring patterns in literature and survey responses. The traffic metering options are detailed in figure 2 5 below while figure 2 4 presents the traffic signal options as classified by Hallworth (1992).
Figure 2 5: Traffic Metering Options at a Signalized Roundabout (Stevens, 2005)
Previous research indicates that signals at roundabouts can improve the overall safety and operation and the general conclusion is that full time operation is a safer option than part time option (Ridding and MacDonald, 2008). Therefore, in this research, a full time approach control on all the entrances of the roundabout is evaluated in order to clearly
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differentiate between unsignalized and signalized cases and quantify the effects of signalization to a roundabout.
2.3 Continuous Flow Intersection
The main purpose of the continuous flow intersection (CFI) is to eliminate left turn conflicts with opposing through movement by displacing the left turn lane to the left side of the road at a distance upstream of the main intersection (Jagannathan, 2004). Right turn movements are designed to bypass the main intersection and are merged back into mainstream traffic downstream. Left turn displacement at the four approaches will create four additional secondary intersections. The innovation of this scheme is the allowance of the operation of both through and left turn movements simultaneously at the main intersection by using a two phase signal (Reid 2001). Figure 2 6 below shows the vehicular movements at a continuous flow intersection.
Figure 2 6: Vehicular movement at a Continuous Flow Intersection (U.S. Department of
Transportation: Federal Highway Administration, 2004)
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The two phase signal operation of the continuous flow intersection, as explained in the Federal Highway Administration’s guide to signalized intersections is illustrated in figure 2 7 where,
a. Street a movements at the major intersection, left turns on the advance
intersections on Street a, and through movements on the advance intersections
on Street B
b. Street a movements at the major intersection and through movements at all
advance intersections
c. Street a movements at the major intersection, through movements on the
advance intersections on Street a, and left turns on the advance intersections on
Street B
d. Street B movements at the major intersection, left turns on the advance
intersections on Street B, and through movements on the advance intersections
on Street a
e. Street B movements at the major intersection and through movements at all
upstream intersections
f. Street B movements at the major intersection, through movements on the
advance intersections on Street B, and left turns on the advance intersections on
Street a
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Figure 2 7: Signal Phasing at a Continuous Flow Intersection (U.S Department of Transportation:
Federal Highway Administration, 2004)
By removing the left turn at the main junction at a Continuous Flow Intersection, the safety benefits of the CFI, when compared to a typical four legged conventional intersection, are evidenced by a reduction of vehicular conflict points from 32 to 30 for the Continuous Flow, as seen in figure 2 8. A four legged signalized intersection contains 16 merging/diverging, 12 left turn crossing, and 4 angle crossing conflict points
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whereas the Continuous Flow intersection has 14 merging, 6 left turn crossing, and 10 angle crossing conflict points.
Figure 2 8: Conflict Points at a Continuous Flow Intersection (FHWA)
The key operational benefit of this intersection is that multiphase signal operation is not required to provide protected left turn movements. Continuous flow intersections provide an at grade spatial solution that can improve traffic operations beyond the capabilities of other conventional at grade solutions (Berkowitz et al., 1996).
Mier, Goldblatt, and Friedman (1994) compared traffic performance at a continuous flow to that of conventional intersections under multi phase control in order to evaluate the effectiveness of the CFI concept. TRAF NETSIM simulation model, which considers each vehicle as a unique entity, was used in the study to model the alternatives. GTRAF, interactive graphic software, was utilized in support of the simulation model to observe vehicles travelling through the intersections with the signal indications changing overtime. Three case studies were considered with demand volumes set at 1,500, 2,000 and 3,000 vehicles per hour respectively on all approaches. Turn movements of 15 percent left turn, 11 percent right turn, and 5 percent trucks were also set equal on all the approaches. The measures of effectiveness used in evaluating the intersections included:
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• Total number of vehicle trips handled by the intersection.
• Total delay time experienced by all vehicles.
• Ratio of time spent moving to the total time spent going through the
intersection
• Average vehicle speed
• Average percentage of available space occupied by vehicles
• Number of times vehicles could not clear the intersection during the green
signal phase
• Number of gallons of fuel consumed.
• Hydrocarbon, carbon monoxide and oxides of nitrogen emissions
• Percent of demand volume served by the intersection
Both the conventional intersection and the continuous flow were able to process the
1,500 vehicles per hour demand. However, increasing the demand to 2,000 vehicles on each direction (4,000 vph total) left the conventional intersection incapacitated by nearly
20 percent while the CFI easily processed the total demand. And although the capacity of the CFI was exceeded when the demand was increased to 3,000 vehicle per hour per direction, compared to the conventional intersection, the CFI had a capacity of nearly 50 percent higher. In addition, the CFI delay time, average speed, and vehicle emissions were substantially better than the conventional intersection with the CFI delay at 1/5 of the conventional, nearly doubled mean speed, an 80 percent increase in signal efficiency, and a reduced vehicle fuel consumption and emissions of more than 1/3.
Reid and Hummer (2002) compared travel time between seven unconventional arterial intersection designs – the quadrant roadway intersection, median u turn, 36
superstreet median, bowtie, jughandle, split intersection, and continuous flow intersection designs. Unlike previous comparisons of intersection delay and travel time between these particular unconventional designs and conventional counterparts where analysis was largely based on hypothetical volumes, Reid and Hummer used turning movement data from seven existing intersections of varying sizes in Virginia and North Carolina to conduct simulation experiments in order to fairly compare the travel time of these intersection designs. For each design, optimum cycle lengths were used and various factors were held constant for a fair comparison. The alternatives were analyzed for off peak, peak, and peak plus 15 percent volume conditions.
Although the quadrant roadway and the median u turn designs consistently produced the best travel times, the continuous flow intersection showed improvement over the conventional intersection in several scenarios. Compared to a conventional intersection, the continuous flow intersection produced travel time of 1 to +25 percent during off peak conditions, and 12 to +27 percent during peak conditions. However, the results of the study showed a general increase in the overall percent of stops at the CFI when compared to a conventional intersection: +21 to +87 percent during off peak conditions, and +12 to +49 percent during peak conditions.
Jagannathan and Bared (2004) assessed the design and operational performance of three different configurations of the continuous flow intersection design (also known as crossover displaced left turn intersection). The first configuration modeled was a four legged intersection with three through lanes per direction, two left turns, and one right turn lane per approach for all four approaches. The displaced left turn lane before the main intersection was 325 feet long while the right turn bay and the left turn bay before
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the separation of the displaced left turn were 250 feet and 350 feet respectively. All the acceleration lanes for the right turning vehicles were 300 feet long. A comparable conventional intersection with similar geometric features was also modeled. Traffic flows on the approaches were randomly generated mostly to replicate peak hour directional flows at intersections. Ranges of 100 to 750, 300 to 2,650, and 50 to 350 vehicles per hour per direction were respectively used for left turn, through movement, and right turn.
The second configuration featured a partial CFI model containing three through lanes per direction, two left turn lanes, and one right turn lane for the two major road approaches. The displaced left turn lane before the main intersection was 350 feet long.
The right turn bay was 275 ft long. The left turn bay before the separation of the displaced left turn was 350 ft long. The acceleration lanes for the right turning vehicles were all 300 ft. The other two legs had the conventional geometric design with two through lanes in each direction, one left turn lane at the major crossing, and one right turn lane. The right turn bay before the intersection was 300 ft long. The left turn bay before the intersection was 350 ft long on the major road. The traffic flows on all the approaches were also randomly generated with a large number of cases modeled with directional flows designed to replicate peak hour directional flows at intersections. The range used for the left turns on the major roads was between 100 and 700 vph while the range for the through traffic on the major roads was 300 to 2,200 vph and the range for the right turning vehicles on the major roads was 50 to 350 vph in one direction. The range used for the left turns on the minor roads was between 50 and 200 vph in per direction. The
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range for the through traffic on the minor roads was 50 to 1,200 vph per direction. The range for the right turning vehicles on the minor roads was 50 to 250 vph per direction.
The third configuration was a T intersection that had three through lanes per direction on the major road and a displaced left turn double lane on the eastern approach, three through lanes per direction, one right lane on the western approach, and two left lanes and one right lane on the T leg on the southern approach. The displaced left turn lane before the main intersection is 325 ft long. The right turn bay is 300 ft long on the western approach. The left turn bay before the separation of the displaced left turn was
350 ft long. The acceleration lanes for the right turning vehicles are 300 ft long. The southern approach had the conventional geometric design with two left lanes and one right turn lane. The left turn bay before the intersection was 350 ft long.
A signal optimization model with the following constraints was developed in order to optimize the signal setting for each of the cases:
• Reduction of delay for vehicles waiting to turn left,
• Reduction of delay for vehicles that had entered the displaced left turn lane
and were traveling toward the main intersection to eventually turn left,
• Reduction of delay for vehicles that had turned left and were traveling to the
final crossing on the through approach,
• Reduction of delay for the through vehicles,
• Reduction in the number of stops for all vehicles, and
• Minimum green time required for pedestrians
Traffic simulation platform VISSIM was used to study the performance of the continuous flow intersections as well as the conventional intersection designs. In order to
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assist planners and traffic engineers in selecting the proper intersection configuration under a wide range of flow conditions, statistical models were developed to estimate two variables of interest commonly used by practitioners in assessing intersection traffic performance. The models were developed using the nonlinear regression technique readily available in the SAS software (Proc NLIN) to express an exponential form. After several trials and iterations of different variables and model forms, the following formulations as seen in figure for predicting average control delay (CD CASE ) in seconds/vehicle and average queue (AQ CASE ) in feet were adopted by the authors:
CD CASE A = exp [ a0 + a1XWN + a2XES + a3XNE + a4XSW + a5XNS + a6XEW + a7XWE
+ a8 X SN ) / 10,000] … … … (2 1)
AQ CASE A = exp [b0 + b1XWN + b2XES + b3XNE + b4XSW + b5XNS + b6XEW + b7XWE
+ b8XSN ) / 10,000] … … … (2 2)
Where,
a and b = regression coefficients with corresponding measures of significance, and
model goodness of fit
XWN = flow from the western approach toward the northern approach (vph),
XES = flow from the eastern approach toward the southern approach (vph),
XNE = flow from the northern approach toward the eastern approach (vph),
XSW = flow from the southern approach toward the western approach (vph),
XNS = flow from the northern approach toward the southern approach (vph),
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XEW = flow from the eastern approach toward the western approach (vph),
XWE = flow from the western approach toward the eastern approach (vph), and
XSN = flow from the southern approach toward the northern approach (vph).
The simulation results proved that introducing the continuous flow intersection on approaches of a conventional intersection design yielded a percentage reduction in average delay of 48 to 85 percent, 58 to 71 percent, and 19 to 90 percent for the first, second, and third configuration respectively. Moreover, compared to the conventional intersections, the continuous flow intersections produced a reduction in the average number of stops of 15 to 30 percent for undersaturated traffic flows and 85 to 95 percent for saturated traffic flow conditions at the conventional intersection.
In their effort to improve the performance of intersections with heavy left turn movements, El Esaway and Sayed (2007) investigated and compared the operational performance of a continuous flow intersection to that of an upstream signalized crossover intersection as well as a conventional design. As previously stated, the continuous flow eliminates left turn opposing conflicts by displacing the left turn lane to the opposing direction and crossing the left traffic to the left side of the road before the intersection.
The upstream signalized crossover intersection, on the other hand, removes left turn opposing conflicts by crossing both the left and through traffic to the left side of the road before the intersection (El Esaway and Sayed, 2007).
The geometries of the three intersections were similar with four legs, two through lanes, one left turn lane of 65 meters in length, and one right turn lane of 90 meters in length. Three geometries were investigated for the continuous flow including separations of 90 meters, 120 meters, and 155 meters between the main and the secondary. Volumes
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were randomly generated to represent peak and off peak conditions as well as balanced and unbalanced scenarios. The effect of left turn movement was also investigated by modeling the unbalanced cases with 20 percent, and 30 percent left turn volume for the same approach volumes. Synchro software was used to optimize the signal timings for the three intersection designs. VISSIM was used to analyze the three intersections. The authors used default parameters with no changes to driver behavior, lane width, grades, or vehicle distributions. Two percent truck was used and an average speed of 50 kilometer per hour (km/h) was assumed for all the approaches. A pre timed signal controller was used with four seconds amber and a one second all red interval for all signals. Five runs with different random seed were performed for each volume scenario.
The analysis revealed that the continuous flow intersection had a lower delay than the other two alternatives under all balanced conditions while the upstream signalized crossover yielded lower delays than the conventional intersection under moderate and high volume levels. Furthermore, the continuous flow intersection had approximately 90 percent higher capacity than the conventional intersection. The continuous flow also outperformed the others under all unbalanced volume conditions.
ABMB Engineers, a Baton Rouge, Louisiana, engineering firm specializing in transportation, has made detailed studies comparing the operational performance of a number of intersections using traffic modeling software. These studies have shown that
CFI designs dramatically outperform conventional alternatives. In case after case, CFI designs produced extraordinary improvements in service levels under existing traffic loads. Average delay was reduced by 90 percent or more. It was also determined that
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these advantages will persist decades into the future, accommodating future traffic growth (Bruce, 2004).
The Federal Highway Administration’s informal guide to signalized intersections
(2004) lists some disadvantages of the CFI, which include:
• Pedestrian acceptance (cross only at main intersection no midblock crossing)
• Driver acceptance (vehicles may be opposed by traffic on both sides).
• Snow removal issues
• Breakdown of vehicles
• Providing access to adjacent parcels
Moreover, footprint of a continuous flow intersection is greater than that of a conventional intersection because it requires right turn lanes and acceleration lanes in each quadrant. In addition, according to Mier, Goldblatt, and Friedman (1994), the construction cost of a CFI may be two to three times the cost of a standard intersection design due to increased right of way costs, and the need for additional, coordinated signal controllers. However, they are significantly cheaper than elevated design interchanges with savings ranging from 5:1 to 10:1 (Berkowitz et al., 1996).
2.4 Parallel Flow Intersection
High capacity intersection, according to Parsons (2007), is achieved by reducing phases, cycle lengths, and conflict points. Safely increasing the capacity of existing urban arterial intersections within confined spaces and at an acceptable cost is an ongoing challenge to transportation engineers and decision makers. The parallel flow or paraflow intersection (PFI) was introduced as a likely solution to satisfy the for mentioned objectives at many intersection locations where improvements are needed. Similar to the
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continuous flow intersection, left turns at the paraflow intersection cross over opposing travel lanes during the cross street through movement phase. Due to protected left turn phases, the larger volume through traffic is allowed to proceed with no lost time by this process of concurrent left turn and through movements. However, unlike the continuous flow intersection, the paraflow achieves this operation with bypass turn lanes parallel to the cross street center lanes, resulting in a smaller intersection with different characteristics (Parsons, 2007).
Wilbur Smith Associates in association with HDR Thompson Transportation
Engineering (2008) prepared an overview as well as implementation guidelines of several unconventional intersection designs including the paraflow, the continuous flow and several others for the Community Planning Association of Idaho. The authors illustrate the key differences between the Paraflow and the continuous flow intersections. One of the main distinctions, as seen in figure 2 10, is that turn pocket storage and transition area are provided in advance of the main intersection in the CFI, whereas the PFI’s transition area is located on the receiving leg of the left turn. In addition, the left turn transition area for the continuous flow intersection is located at the main intersection where a driver performing a right turn at a conventional intersection might enter. To remedy that situation, a separate right turning lane must be provided before reaching that left turn pocket in order to discourage the potential for wrong turns at the left turning bay. This additional lane, therefore increases the footprint of the CFI, making the paraflow’s footprint a better fit with existing adjacent land uses and a preferred alternative in situations where lack of available space is a decisive constraint.
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Figure 2 9: Comparison of Typical CFI and PFI Footprints and Turning Paths
(Red illustrates left turns and cyan illustrates right turns) (Quandrant Engineering, 2009)
The paraflow operates with a signal cycle, which consists of two phases as illustrated in figure 2 11. The main junction signals must be coordinated with the bypass junction signals based on traffic volume distribution and distance between junctions. In the first phase, drivers pass through the approach bypass junction and proceed into the center left turn lane, then are stopped by the main junction left turn signal. In the second phase, left turning drivers turn onto the bypass roadway and continue to the departure bypass junction. Right turning drivers pass through the approach bypass junction by traveling onto the bypass roadway and then merging onto the departure roadway. Through movements follow the same travel pattern as a conventional intersection, passing through the three coordinated signalized junctions (Parsons, 2009)
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Figure 2 10: Vehicular Movement at a Parallel Flow Intersection (Quadrant Engineering, 2009)
The paraflow intersection design provides the benefit of safety over conventional four legged intersections through a reduction of conflict points. Compared to a conventional intersection, which typically encompasses 32 conflict points: 16 merging/diverging, 12 left turn crossing, and 4 angle crossing, the paraflow intersection has only 28. Figure 2 12 exemplifies the comparison of conflict points between a paraflow intersection design and a typical conventional intersection. Other than the reduction of conflict points, the paraflow protects and removes all turn movements from the main intersection, which significantly decreases the potential for accidents (Parsons,
2007).
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Figure 2 11: Comparison of Conflict Points between a Conventional Intersection and a Parallel Flow
Intersection (Parsons, 2009)
The available literature emphasizing the operational benefits of the parallel flow intersection compared to other intersection designs: conventional or innovative, is somewhat lacking. Since the paraflow is a newly introduced concept, few researchers have evaluated its performance. As part of his endeavor to establish the parallel flow intersection as a viable alternative to the conventional intersection design, Parsons (2007) analyzed and compared the performance of a PFI to a CFI, a three lane modern roundabout, and a conventional intersection.
The paraflow model in the study consisted of four approaches with two through lanes on each approach. The basis of this analysis was expressed in terms of Level of service and average delay for an intersection with high entrance volume. The total traffic demand considered was 6,375 vehicles per hour with a 30 percent left turning volume.
The total traffic demand was distributed such that a major road was considered with 55 percent directional volume. VISSIM microsimulation platform version 4.10 was utilized
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to model the paraflow and the continuous flow intersections while RODEL and Synchro were used to evaluate the roundabout and the conventional intersection respectively.
The results of the comparison indicated that the paraflow and the continuous flow attained a level of service C for the given volume considered in the study, while the roundabout and the conventional intersection produced a LOS E. The delay results for the paraflow and the CFI were similar with 80 percent and 75 percent delay reduction from the conventional intersection and the roundabout respectively. Given that different methodologies were used to evaluate the intersection designs and that, the different software results are not directly compatible, according to Parsons, the trial results were used solely to give a general indication of relative performance.
Cheong et al., (2008) compared the performance of a parallel flow intersection, a continuous flow intersection, and an upstream signalized crossover. The average delays of through only traffic and left turn only traffic were determined and compared for each intersection. Both balanced and unbalanced flow conditions were studied. For the balanced traffic condition, the approach volume for each unconventional intersection was set to be 1000 vph as the low volume level, 1,500 vph as the moderate level, and 1,800 vph as the high volume level. As for the conventional design, 1,000 vph and 1,200 vph were tested. The percentage of right turn volume was fixed at 10 percent and 5, 10, 20, and 25 percent of left turn volume was considered in order to study the effect of left turn volume on each intersection design. For the unbalanced condition, the approach traffic volume of the main arterial road was set as 2000vph representing the moderate volume level and 2500vph as the high volume level. The approach traffic volumes considered for
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the minor crossroad were 600vph as the low volume level, 900vph as the moderate volume level and 1300vph as the high volume level.
VISSIM version 4.1 was used to analyze all the intersections. The default parameters of the microsimulation platform were used. Truck percentage and speed on all the approaches were assumed as 2 percent and 50 km/h. Travel time detectors were placed upstream and downstream of the main intersection for better capture of the delays caused by the intersection.
All three unconventional intersections outperformed the conventional design.
Among the unconventional intersections, the CFI outperformed the others except for some traffic conditions. For instance, in the balanced traffic condition scenario, at the low traffic volume level, the average delays of through traffic for the Paraflow intersection were smaller than that of the CFI and very similar at the moderate traffic volume level. In the unbalanced traffic condition scenario, under some traffic conditions, PFI outperformed CFI or showed very similar average delay with CFI.
2.5 Intersection Analysis – Analytical and Empirical Modeling
The Highway Capacity Manual (Transportation Research Board, 2000) describes several methods for analyzing the capacity and level of service (LOS) of various types of transportation facilities including intersections. There exist two types of intersection analysis models according to the Highway Capacity Manual: analytical and empirical models. Analytical models estimates capacity following gap acceptance relationships that do not require observations under congested conditions, while empirical models apply observations at different intersections under all conditions to develop regression formulations that match intersection characteristics with the intersection capacity (Stanek
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et al., 2004). Level of service is an essential quality measure that describes the conditions of the traffic entering the intersection in terms of comfort and convenience for the users.
Separate methodologies are provided by the Highway Capacity Manual (HCM) to analyze unsignalized intersection as well as signalized intersections. These methods have been widely used by transportation engineers in the field to assess the operational performance of different types of intersections. Delay is the measure of effectiveness used to determine level of service at an intersection. The procedures for delay and LOS estimation as illustrated in the Highway Capacity Manual are briefly described in the following sections.
2.5.1 HCM Methodology for Unsignalized Intersections
The input data required for the analysis of unsignalized intersections based of the
HCM methodology include detailed description of the intersection’s geometries such as the number of lanes in each approach, the approach grade etc. In addition, the traffic volumes for each movement at the intersection need to be specified.
The control delay at an unsignalized intersection, which includes delay due to initial deceleration, queue move up time, stop delay, and final acceleration delay, is defined as the total elapsed time from the time a vehicle stops at the end of a queue to the time that the vehicle departs from the stop line (Boddapaty, 2008). The Highway Capacity manual provides the formulation used to estimate control delay as seen in figure 2 1. The control delay for any movement at the intersection is a function of the each particular approach capacity and the degree of saturation.
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3600 V X 2 3600 V V cm,X cm,X X X d = + 900T −1+ −1 + + 5 … … … (2 3) cm,X cm,X cm,X 450T
Where,
d = Control delay (s/veh)
VX = Flow rate for movement x (veh/hr),
cm,X = Capacity for movement x (veh/hr) and,
T = Analysis time period (h) (T = 0.25 for a 15 min period)
The level of service for each movement can be determined from the calculated control delay. Based on the values obtained for the average control delay of the intersection, level of service ranges from A to F, where a LOS A indicates a low average control delay, typically between 0 10 seconds per vehicle while a LOS F signifies an average control delay of more than 50 seconds per vehicle (Transportation Research
Board, 2000). Table 2 1, acquired from the Highway Capacity Manual, illustrates the level of service criteria for unsignalized two way stop controlled intersections.
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Table 2 2: Level of Service Criteria for Unsignalized Intersections
Level of Service Average Control Delay
(s/veh)
A 0 10
B >10 15
C >15 25
D >25 35
E >35 50
F >50
2.5.2 HCM Methodology for Signalized Intersections
The Highway Capacity Manual provides a methodology to determine the level of service for a signalized intersection. As with the case of unsignalized intersections, some input data including the geometric features of the intersection, volume conditions, and signal control settings, are required from the user. The geometric features required include number of lanes for each approach, the average width of the lanes, the length of the storage bays etc. Traffic demand volumes for every movements at the intersection as well as saturation flow rate, peak hour factor (PHF), vehicle arrival rate and speed are some of the basic traffic condition information needed. Traffic signal control setting information includes cycle length, green time, and yellow plus all red clearance intervals.
The type of signal control operation, for instance pre timed or actuated, also influences the process of determining the level of service of signalized intersections. 52
The average control delay for a given vehicle at signalized intersections can be determined from the following formulation as illustrated in the Highway capacity
Manual:
d = d1 (PF )+ d 2 + d3 … … … (2 4)
Where,
d = Control delay per vehicle (s/veh)
d1 = Uniform control delay assuming uniform arrivals (s/veh)
PF = Uniform delay progression factor, which accounts for effects of signal
progression
d2 = Incremental delay to account for effect of random arrivals and
oversaturation queues, adjusted for duration of analysis period and type
of signal control; this delay component assumes that there is no initial
queue for lane group at start of analysis period (s/veh) and
d3 = Initial queue delay, which accounts for delay to all vehicles in analysis
period due to initial queue at start of the analysis period (s/veh)
The level of service for the intersection is determined based on the average control delay per vehicle. From the delay model shown in equation 2 4, the average control delay for the intersection can be estimated. Using the average control delay estimated for the intersection, the level of service can be determined based on the criteria provided in the Highway Capacity Manual, as shown in Table 2 3 below.
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Table 2 3: Level of Service Criteria for Signalized Intersections
Level of Service Average Control Delay
(s/veh)
A 0 10
B >10 15
C >15 25
D >25 35
E >35 50
F >50
2.5.3 Roundabout Analysis
Roundabouts operate differently than conventional intersections whether unsignalized or signalized. The methods described in the above sections from the
Highway Capacity Manual, therefore, do not apply in analyzing the operations of a roundabout. The Highway Capacity Manual provides a methodology to estimate the approach capacity at roundabouts using gap acceptance. This method, however, can only be used to analyze single lane roundabouts. Moreover, contrary from the above cases of unsignalized and signalized conventional intersections, methodologies to determine delay and level of service at roundabouts are not provided (Stanek et al., 2004). The National
Cooperative Highway Research Program (NCHRP), a division of the Transportation
Research Board, recently published a study based on roundabouts located throughout the 54
United States (National Cooperative Highway Research Program, 2007). According to
NCHRP’s findings, the roundabout operation is different than a two way stop control unsignalized intersection in that all drivers entering the roundabout have to yield to conflicting traffic and that drivers performing a left turn have to find a gap in only the one direction of travel. However, the NCHRP recommends using the same threshold as two way stop intersections to determine the level of service at roundabouts as seen in
Table 2 2.
2.6 Traffic Simulation Modeling
Simulation modeling provides researchers and transportation engineers the means to remotely study and analyze traffic by accurately modeling field conditions. Based on the specific purpose of the analysis, traffic can be modeled on three different scales: macroscopic, mesoscopic, and microscopic. Separately, each of these scales of simulation model offers its own benefits and shortcomings; however, the advancement in traffic simulation and the need for reliability in the models created have propelled the integration of the three models, in order to accurately represent traffic (Burghout et. al.,
2005).
In macroscopic simulation, traffic is regarded as having the same properties as fluids and thus represented as a continuous flow and modeled using hydrodynamics and fluid flow formulae (Lighthill et al., 1955). Macroscopic models are simplistic; require minimal inputs from users and consequently easier and less time consuming to calibrate
(Lerner et. al., 2001Burghout et. al., 2005). However, the disadvantage of macroscopic models is that its simplistic nature renders it incapable of accurately representing
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complex management systems and geometries and in depth traffic control (Owen et. al.,
2000).
Mesoscopic simulation models comprise characteristics of both macroscopic and microscopic models in that individual cars can be modeled to a certain degree. In a mesoscopic simulation model, every vehicle is individually represented; however, behaviors of each separate vehicle such as car following or lane changing are not modeled using detailed interaction between cars but are rather generalized and modeled using density of the lanes (Lieberman, 1997).
Whereas macroscopic simulation models are simplistic and less time consuming in the building and calibrating process, microscopic simulation requires a significant amount of inputs and very time consuming to build and calibrate the models (Lerner et. al., 2001). Unlike macroscopic and mesoscopic simulation models, microscopic models are able to accurately and intensively model individual car and driver behavior. Using microscopic simulation platform, one is able to analyze vehicle and driver interaction including lane changing, car following, response to traffic incidents, as well as change in the road geometry (Burghout et al., 2005). Microscopic simulation provides a visual representation of problems and solution in a way a layman or a professional in the transportation field can understand and can also be a powerful tool to gain widespread acceptance of complex strategies (Halcrow, 2003). This research evaluates the performance of several isolated intersection designs. The need to analyze vehicle and driver interaction such as lane changing, car following is critical and can only be assessed on a microscopic level. Hence, microscopic platforms are used to simulate and study each intersection design.
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2.7 Microsimulation versus Analytical/Empirical Modeling
Analytical and empirical modelings are the traditional means of analyzing the performance of intersection designs – unsignalized or signalized. In both cases, mathematical formulations are developed to provide estimates of capacity and other performance statistics. In recent years, microsimulation has become an accepted transportation planning tool; however, it has yet to establish the same kind of credibility as traditional traffic models (Austroads, 2006). Researchers and transportation planners widely use both methods of analysis although there is still an ongoing debate about the merits of each type of tools. According to Algers et al. (2000), microsimulation is useful but dangerous. Akcelik (2007) identified several weaknesses of microsimulation models, which include:
• Substantial data requirements
• Results easily influenced by the modeler
• Calibration difficulties
• Benchmarking and comparisons can be difficult
• The realistic graphical representations can give unrealistic expectations
of accuracy that isn’t there
Many researchers have compared results obtained from microsimulation to other traditional analysis tools. The capacity analysis suggested in the Federal Highway
Administration’s roundabout guidelines was compared to results of the analysis software packages RODEL, aaSIDRA, VISSIM, AND Paramics (Stanek and Milam, 2004).
RODEL is an empirical macroscopic analysis model developed and distributed by Barry
Crown based on many observations in the United Kingdom. Distributed by Akcelik and
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Associates, aaSIDRA is an analytical model that applies Australian methodologies similar to the Highway Capacity Manual methodologies. Paramics and VISSIM are microscopic simulation platforms that use individual driver behavior and vehicle characteristics to model traffic operations to provide performance measures. A diamond interchange with roundabouts located in California was used as a case study in the comparison analysis. The results indicated that the microsimulation models provided
‘more accurate and reasonable results.’
Moreover, Bared and Edara (2005) compared microsimulation results from VISSIM to the empirical model RODEL and analytical model aaSIDRA in their analysis of single lane and double lane roundabouts as described previously. The results from each model were compared to real data collected from various sites in the United States. The authors found that the VISSIM results were closer to the real data than the RODEL and aaSIDRA results.
These example studies demonstrate that, when used properly, microsimulation models can produce results that are accurate and better emulate field conditions comparing to traditional intersection analysis models, which in turn, validate the use of microsimulation to evaluate the performance of the particular unconventional intersections considered in this thesis.
2.8 Comparison of Microsimulation Platforms
In order to select a microsimulation platform capable of analyzing the operational performance of the intersection designs being evaluated in this thesis, an extensive literature review was conducted on the comparison of available microsimulation models.
Several studies have compared different microsimulation platforms; however this section
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reports past research related to microsimulation software comparison with respect to intersection analysis. Some of the key criteria, on which these comparisons were founded, include but are not limited to:
• Data requirements; appropriateness of defaults
• Difficulty / ease of coding
• Capability of representing specific geometric characteristics
• Capability of simulating specific signal control plans
• Relevance / accuracy of performance measures reported in the output
Fang and Elefteriadou (2004) studied and compared three widely used traffic simulation packages: AIMSUN (TSS, 2002), CORSIM (FHWA, 1997), and VISSIM
(PTV, 2002). The comparison was carried out based on four particular elements: (1) the capability of representation of specific geometric characteristics; (2) the capability of simulating specific signal control plans; (3) calibration needs and accuracy of in comparison to field conditions; (4) extraction of performance measures from the simulator. In order to achieve their goal, the authors obtained data from two intersection designs located in Arizona. The two intersection designs selected as case studies were a diamond interchange and a single point urban interchange (SPUI). Simulation was first conducted by using default parameters from each model. Then, the models were calibrated to adjust parameters related to driver behavior and vehicle performance. Delay was chosen as the measure of effectiveness to compare the simulation output.
Regarding the capability to represent geometric characteristics, the authors found that all three simulation platforms were capable of representing urban streets and freeways including the ramps required for the interchanges. As regards to the capability
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of simulating signal control, CORSIM had difficulties to represent the SPUI when frontage roads exist whereas the other models revealed no issues. Since the definitions of performance measures differ among models, it was difficult to calibrate each simulator in order to compare their outputs to field results. The authors recommended that users should use caution and modify values within a reasonable range. Finally, in terms of provision of performance measures, all the simulation platforms produce a variety of necessary measures including delay, travel time, speed, and number of stops, which for the most part, are comparable within models.
Detailed features of four major traffic simulation models: CORSIM, SIMTRAFFIC,
Paramics, and VISSIM, was presented in the work of Park et al. (2004). The performance of these models was evaluated for a coordinated signal system in terms of response to various timing plans, stochastic variability and simulation run time. In order to provide reference performance measures for particular traffic signal settings including pre time and actuated controls, the signal optimization software Synchro was used.
It was found that CORSIM and SIMTRAFFIC contain network limits, while Paramics and VISSIM do not. However, in the case where coordinated and actuated traffic signal timing plan were evaluated, Paramics required additional programming work for actuated control logic. Comparing trends from the simulation models to that of Synchro revealed that VISSIM and Paramics showed consistent performance trends to all signal timing plan cases whereas CORSIM and SIMTRAFFIC produced incoherent performance trends especially for optimized timing plans.
Jones et al., (2004) performed a comparison study of several traffic simulation platforms for the Regional Planning Commission of Greater Birmingham, Alabama. The
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three microsimulation packages selected for evaluation were CORSIM, which according to the authors is “the most widely used microsimulation program in the US,” SimTraffic, and AIMSUN. The criteria used as the basis for this study were ease of coding, visualization capabilities, and reliability of outputs. The models were tested for several case studies and performance measures such as link speed, travel time, and delay time were compared. The authors concluded that each model had strengths and weaknesses that made it suitable for certain applications, depending on the type of transportation improvement or planning analysis being considered. All the models are able to simulate street networks, interchanges, stop controlled intersections, and roundabouts; however, the authors note that AIMSUN possesses capabilities beyond either CORSIM or
SimTraffic.
AIMSUN and VISSIM are “two of the best regarded simulators” available on the market (Xiao et al., 2005). Both models have the capabilities of modeling complex networks as well as various types of interchanges and are therefore used in this research to evaluate and compare the performance of the selected intersection designs.
2.9 Research Objectives
The objective of this research is evaluate and compare the operational performance of four unconventional intersection designs: unsignalized roundabout, signalized roundabout, continuous flow intersection, and parallel flow intersection using microscopic simulation. These unconventional intersections will also be compared to a conventional design. The results of this evaluation will assist transportation professionals in selecting an appropriate alternative to the conventional intersection design based on
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particular situations. The specific tasks needed to attain the objectives of this research are as follows:
• Develop a model of each intersection alternative with comparable
geometric features
• Develop traffic demand data to emulate field conditions
• Perform a detailed statistical analysis of the performance measures
extracted from microsimulation platforms.
To accomplish these tasks, AIMSUN and VISSIM microsimulation platforms are selected to model the intersections and provide performance measures, which will be the basis of the comparison analysis. Traffic demand data are randomly generated to represent peak and off peak conditions as well as balanced and unbalanced flow conditions. In addition, different scenarios are created to quantify the effect of left turn movements on each intersection. Performance measures are selected based on their relevance vis à vis the operational evaluation of intersection designs.
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3 METHODOLOGY
The remainder of this thesis consists of four major components. The first component describes the required input data for the simulation analysis including the geometric characteristics of each of the intersections, the traffic demand data, and the signal timing optimization procedure for each intersection design. The measures of effectiveness used in this research as well as measures of effectiveness from previous studies are described in order to substantiate the selection of the specific performance measures used to compare the different alternatives. Second, the simulation procedures in AIMSUN and
VISSIM are described in terms of their specific algorithms used to model traffic. The measures of effectiveness from each model are defined to establish the foundation for comparison between the two simulation packages. The procedure for developing the models for each intersection designs is detailed in the simulation studies chapter. Once performance measures are extracted from the microsimulation platforms, a statistical analysis is performed to compare the chosen intersection designs as well as the performance of the simulation platforms. Conclusions are drawn from the results and analysis section. The major limitations of the study and recommendations for future work are also elaborated in the conclusions section. The following flowchart (Figure 3 1) illustrates the methodology used in this research:
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Figure 3 1: Methodology Flowchart
3.1 Geometric Elements
To conduct a fair comparison, the geometric features of all the intersections were ensured to be similar. For the five intersection designs: conventional, unsignalized roundabout, signalized roundabout, continuous flow, and parallel flow, the following lane configurations were used: all intersections have four approaches; each intersection has the same number of lanes per approach. Aside from the roundabouts, which have different characteristics, all the intersections have one left turn lane and one right turn lane per approach.
For the continuous flow intersection and the parallel flow intersection, the distance between the main junction and the sub intersections had to be determined. The capacity of the intersection is dependent on that particular distance since it also affects the capacity of storing the left turn traffic. Many researchers including Jagannathan and
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Bared from the Federal Highway Administration have suggested that the distance between the main and the sub intersections should be within 300 to 500 feet long in order to be able to operate efficiently (Pitaksringkarn, 2007; Cheong et al., 2008).
For both the balanced and unbalanced scenarios, the distance between the main and sub intersections is set at 325 feet for the continuous flow intersections. In addition, the right turning pockets are set to be 250 feet and the acceleration lanes are set to be 325 feet. See Appendix A for a detailed AutoCAD drawing of the continuous flow and the parallel flow intersections.
In order to maintain the equivalent characteristics for all the intersection designs, the left turn and the right turn lanes for the conventional intersection are 325 feet and 250 feet, respectively. Figure 3 2 presents the geometric elements of the conventional intersection.
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Figure 3 2: Geometric Elements of Conventional Intersection AutoCAD Drawing
As for the double lane roundabout networks, the geometric features, as recommended by the Federal Highway Administration’s roundabout guide in order to achieve safe and efficient operation, are presented in Table 3 1 (Bared and Edara, 2005).
The inscribed circle diameter is 180 feet (55 m); the entry and exit radii are approximately 131 feet (40 m); the approach and departure width are set at approximately
24 feet (7.3 m); and the circulatory road width is 31 feet (9.5 m).
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Table 3 1: Geometric Elements of the Modeled Double lane Roundabouts
Geometric Elements Measurement (meters)
Inscribed Circle Diameter 55
Entry Radius 40
Exit Radius 40
Entry Width 40
Approach Width 8.5
Departure Width 7.3
Exit Width 7.3
Circulatory Road Width 9.5
3.2 Traffic Demand
The traffic demand data required for the analysis of the intersection schemes were generated hypothetically with the consideration of three critical issues. First, each intersection was evaluated under different volume levels ranging from low to high in order to simulate both peak and off peak conditions. The second issue considered was testing the intersection designs under balanced and unbalanced flow condition. Under a balanced flow condition, the volumes on all four approaches are similar whereas an unbalanced volume condition considers a major and minor road at the intersection. The third issue was to quantify the effect of increasing left turn volume on the performance of the intersection.
To emulate peak and off peak traffic data that can be observed in the field, volumes ranging from 1,000 to 6,000 vehicles per hour were tested for each intersection designs.
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These volumes were chosen because typically in the field, 1000 vehicles per hour represent low entering flow at an intersection during off peak period whereas 6000 vehicles per hour represent high flow situation where conventional intersections are performing close to saturation. These volumes were generated based on field trends and works of previous researchers. Bared, Edara, and Jagannathan (2005) used similar volumes with low (1,450 vph), medium (2,890 vph), high (5,410 vph), and peak (5,752 vph) entering volumes to compare the operational performance of several unconventional intersections. Volumes were selected at an increment of 500 vehicles per hour for ten different scenarios. The 500 vehicles per hour increment helps to account for the minimal changes in the intersection performance between the off peak and peak conditions. These hypothetical volumes were chosen to accentuate the capacity difference between the intersection designs in order to specify which intersection provides a better alternative to the conventional intersection for low, medium, and high flow conditions.
For the balanced flow conditions, the volumes are assumed equivalent on all the entrances. The left turn volumes (L), the through volumes (T), and the right turn volumes (R) for Eastbound, Westbound, Northbound, and Southbound are identical.
Flows from 1,000 vehicles per hour to 6,000 vehicles per hour are considered with the increment of 500 vehicles per hour.
For the unbalanced scenario, the volumes are divided between the four approaches of the intersection with consideration of a major and a minor road with 70% of the total volume on the major road and 30% on the minor road. The volumes on the major road were redistributed to account for the unbalanced condition. Sixty percent of the volume
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on the major road was assigned to the westbound entrance and the remaining forty percent to the eastbound entrance. On the minor road, the volume was distributed evenly on the North and South approaches. Moreover, three unbalanced flow scenarios were adopted from the work of Bared and Edara (2004) in order to compare the results obtained in VISSIM to that in AIMSUN. These traffic demand volumes were obtained from a site in Missouri during peak hour then increased by twenty five and fifty percent to represent low, medium, and high entering flow at the intersections.
Three different scenarios with 15%, 20%, and 25% left turn volume were modeled to quantify the effect of left turn volume on the intersections. These left turn percentages were selected to represent different field scenarios. The fifteen percent left turning scenario is commonly observed in the field; however, in certain cases in the field, higher left turning percentages may be encountered. Thus, the twenty and twenty five percent left turning scenarios are chosen to represent these instances of higher left turning percentages. The different volumes generated for the analysis are presented in the simulation studies chapter.
3.3 Optimization Models
To obtain the optimum signal settings for each of the intersections, several methods were considered. First, a mathematical optimization model was developed based on
Webster’s proposed methodology for optimizing signals for isolated intersections, which was derived from the theory of stage and cycle lengths duration. Webster (1958) derived the first approximate expression for the delay at an intersection and his contribution has been the basis for this research field as his derived expression is still widely used and constantly improved by other researchers (Osorio 2008). Since then, many other
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researchers have formulated their own expressions in attempting to minimize delay at isolated intersections. Among those, include A. J. Miller (1963) and D. R. McNeil
(1968) who considered the effect of compound Poisson arrivals in his solution to the fixed cycle traffic signal problem. Dion et al. evaluated and compared different delay models at both under saturated and over saturated conditions and found that the delay computed from Webster’s delay formulation is comparable to other models as well as results calculated from field data (Dion 2004). The mathematical model developed in the attempt of determining optimum signal settings for the intersections is briefly described below.
FORMULATIONS
In order to formulate the signal setting problem, the following notations are introduced:
D: average delay at intersection (s/veh)
C: Cycle length (sec) q: Total entering flow (vph) qi: flow for each phase (vph)
S: saturation flow rate (vphg) x: degree of saturation yi: flow ratio for each phase (qi/si)
gi: green split time for each phase (sec) (green time of phase i divided by the cycle
time of the intersection) g: effective green time for intersection (sec) li: lost time for each phase (sec)
λ: ratio of effective green time to the cycle time
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The problem is formulated as follows:
Minimize:
C 1( − λ) 2 x 2 D = + − .0 65(C / q 2 ) 3/1 x 2+5λ 1(2 − λx) 2q 1( − x) … … … (3 1)
Subject to
G λ = … … … (3 7) C
… … … (3 6) G = ∑ gi
y (C − l ) g = i ∑ i … … … (3 5) i ∑ y
… … … (3 9) gi > 10
… … … (3 4) li = 6
v y = … … … (3 3) s
x < 1 … … … (3 2)
… … … (3 8) 60 ≤ C ≤ 120
The objective is to minimize the average delay in an isolated intersection which is represented by D (equation 1). The expression for D was derived by Webster (1958) and has been the basis of all delay minimization problem for isolated intersections in the traffic study field. Since then, more complicated formulations have been developed but it’s been proven that their results when compare to those of Webster’s are only marginal.
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In some cases, where only the entering flow data are considered, Webster’s formulation yields better results comparable to observed delay in the field. Therefore, it is applicable for the purpose of this paper. The linear constraints (3) link the green times for each phase with the available cycle time for the intersection. Equation 4 computes the green splits for each phase of the intersection linking the flow ratio for each phase with effective green time of the intersection (cycle time C minus the sum of lost time for each phase). Equations 5 and 6 constitute the bounds for the minimum green time and the lost time for a phase, respectively. These bounds are based on the standards taken from the
Highway Capacity Manual. This problem is formulated for cases where the intersection is performing at under saturated conditions, as seen in equation 8 (x < 1), otherwise,
Webster’s formulation may yield an overestimation of the delay at the intersection.
Equation 9 bounds the solution space for the cycle time based on the minimum and maximum cycle length for a phase requirement from the Highway Capacity Manual.
To test the model, a conventional intersection with two lanes in each approach was used, as seen in figure 3 3. Direction N S is considered the major arterial and direction
E W is considered the minor arterial. The intersection consists of eight phases with separate left turn movement and a combination of right and through movement (see figure 3 4). Following the Ring and Barrier guideline from the Federal Highway
Administration, the phases were organized by grouping them in a continuous loop and separating the conflicting traffic streams with time between when they are allowed to operate, by making the movements sequential. A barrier is used to separate the east west movements from the north south movement to avoid operating at the same time and to
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define a relationship between the rings to assure compatible movement. As seen in figure
3 5, the phases are grouped as follows:
• Phases 1, 2, 3, and 4 are assigned to Ring 1. Phases 5, 6, 7, and 8 are assigned to
Ring 2.
• Phases 1, 2, 5, and 6 are assigned to Barrier 1. Phases 3, 4, 7, and 8 are assigned
to Barrier 2
The sequence of phases is shown as they occur in time, proceeding from left to right.
The figure illustrates a phase sequence with left turn movements leading the opposing through movements on both the major and minor streets. The diagram (figure 3) shows phase 1 and 5 ending at the same time, but they operate independently and can end at different times. The subsequent phase (phases 2 and 6 respectively) may begin once the previous phase has used its time. Once the barrier is crossed phases 3 and 7 operate followed by phases 4 and 8. The cycle ends with the completion of phases 4 and 8.
Figure 3 3: Two lane Conventional Intersection Applied in the Case Study to Test the Model
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Figure 3 4: Vehicular Movement at the Intersection (FHWA, 2004)
Figure 3 5: Phase Grouping for the Case Study Intersection Following Federal Highway
Administration Guidelines (FHWA, 2004) 74
A total entering flow of 4000 vph was used in the case study. The flow distribution between the four approaches of the intersection was unbalanced with 70% of the flow going to the approaches on the major street (N S) and 30% on the minor street. The vehicular flow distribution is presented in Table 3 7. A 15% left and right turning percentage with 70% through was used in the case study.
Table 3 2: Vehicular Flow Distribution Applied to Test Signal Optimization Model
Entering Flow (vph)
Total Northbound Southbound Westbound Eastbound
4000 1400 1400 600 600
After obtaining all the geometrical and phase assignment inputs from the intersection, the model was applied and the problem was solved using Excel solver.
Within the solver options tab the values for the max time; convergence etc. was adjusted in order to test different option. A max time of 100 seconds, with 100 iterations, a precision of 0.000001, a 5% tolerance and a convergence of 0.0001 was used. The solver was also set to assume non negative numbers for all values. Then, after setting the constraints, Excel solved the problem, providing the different green times for each phase, the cycle time, as well as the optimum delay time for the intersection.
Excel found an optimal cycle length of 90 seconds with effective green splits of 16, 31,
11, and 13 seconds for the 1 5, 2 6, 3 7, and 4 8 combinations respectively, and a delay time of 21.88 seconds.
In order to validate the results found from the excel solver, the signal timing for the same intersection used in the case study was optimized using PASSER II – 02 and
TRANSYT 7F. PASSER II 02 minimizes delay for an intersection using the delay 75
formulation from the Highway Capacity Manual. PASSER II uses a bandwidth optimization technique, which is a numerical search technique known as Interference
Minimization. The reader is referred to Messer, et al., HRR 445 for a complete description of PASSER II optimization procedure. To search for the optimum solution,
TRANSYT 7F uses a genetic algorithm. Refer to the TRANSYT 7F manual for a better understanding of the optimization procedure used in the package.
Once the required data were imputed in the programs, they searched for an optimal solution and generated the optimum cycle length and the optimum splits. The results obtained from TRANSYT 7F were discarded because the program generated solutions consisting of a minimum of 6 phases for every attempt. This can be because the program was designed with the purpose of mostly optimizing networks of intersection rather than isolated intersections. The results obtained from PASSER II as well as those from Excel
Solver were imputed into an AIMSUN model of the case study intersection. The results show that the signal settings from PASSER II provided lower delays than the developed model. Moreover, the delay values obtained from using the signal control plans generated by the developed model were comparable to that of PASSER II. However, because the problem formulated was non linear, Excel Solver fails to provide a solution on several occasions. Therefore, PASSER II – 02 was used to optimize the signal timings for the conventional intersection design and the signalized roundabout.
Researchers from the Federal Highway Administration (2004) have optimized the signal timing plans for a continuous flow intersection with geometries similar to the intersection being evaluated in this research. The various flows considered in that particular study were similar to the ones considered in this thesis. The optimized signal
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settings control and the pre timed phasing scheme implemented for the continuous flow intersection is displayed in figure 3 6. As the parallel flow intersection is similar to the continuous flow intersection in that they both have similar geometries and they operate in the same way, the signal timings considered for the continuous flow were implemented for the parallel flow intersection.
Figure 3 6: Signal Timing Control Settings for the Continuous Flow Intersection (Jagannathan and
Bared, 2004)
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4 SIMULATION STUDIES
As previously stated, Aimsun 6.0 and VISSIM 5.10 were used to analyze all experimental designs. In this section, each of the microsimulation platforms used are briefly described in terms of how traffic networks are created and modeling of vehicle movement. Moreover, the procedures used to generate the intersection models are briefly explained. The simulation scenarios used to simulate all the intersections are also presented in this section.
4.1 Aimsun 6.0 Simulation Platform
AIMSUN, developed at the Universitat Poletecnica de Catalunya, Barcelona, Spain, is a microscopic simulator that is able to recreate real traffic conditions of different traffic networks on a computer (AIMSUN User Manual, 2008). The platform uses object oriented simulators and a graphical user interface (GUI) to produce both 2D and 3D animations of the road network. In AIMSUN, the behavior of every single vehicle is continuously modeled throughout the simulation period using several driver behavior models such as car following, lane changing, and gap acceptance. The reader can refer to the AIMSUN user’s manual for more a complete description of these algorithms.
AIMSUN uses links and nodes to generate a traffic network. The model is capable of distinguishing between different types of vehicles; modeling traffic demands based on traffic flows and turning proportions or origin destination (OD) matrices; modeling
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different traffic control types including fixed time control, actuated and adaptive control
(Xiao et al., 2005). Different object features such as segments, nodes, centroids, meters, detectors etc. are used in the model to accurately evaluate a wide range of traffic networks. Another important feature of Aimsun 6.0 is the ability to import networks created from different platforms such as GIS, AutoCAD, CONTRAM, Paramics,
Synchro, and VISSIM among others (AIMSUN User Manual, 2008). The model generates measures of effectiveness that is typically used in the field such as delay time, travel time, number of stops, fuel emissions, queue length etc.
4.2 VISSIM 5.10 Simulation Platform
VISSIM, developed by German company Planung Transport Verkher (PTV), is a microscopic, time step and behavior based simulation platform developed to model urban traffic operations (VISSIM User Manual, 2008). VISSIM simulation operates through two different phases and consists of an online and offline component. The simulation generates an online visualization of traffic operations and an offline generation of output files gathering statistical data such as delay times, travel times etc. The traffic simulator in VISSIM is a microscopic traffic flow simulation model able to model the behavior of every single vehicle through car following and lane changing logic. Refer to the VISSIM user manual for a detailed description of the traffic simulation model.
VISSIM uses a link and connector system to create a network. The platform models traffic considering a variety of inputs including lane assignments and geometries, traffic demands, which can be based on flow rates and turning percentages by different vehicle types or origin destination matrices, acceleration and deceleration, signal control plans: fixed, actuated, or adaptive control. Measures of effectiveness generated from the model
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may include delay and travel times, fuel consumption and emissions, number of stops, as well as queue length.
4.3 Calibration and Validation
Calibration is the process of software user to adjust model parameters in order to reproduce local behavior and meet local field data (Fellendorf, 2004). Calibration of any simulation models is essential in ensuring that the model accurately represents field conditions. In the calibration process parameters of the simulation model are adjusted to match observations in the field. This research however, compares several intersections that are still in the testing and development phase and field data are not yet available.
Moreover, this research aims to compare the results obtained from AIMSUN to those obtained from VISSIM based on the individual model’s default parameters and driver behavior logic. Parameters such as speed, gradient, and geometries are the same between
AIMSUN and VISSIM to guarantee a fair comparison; however, default values for driver behavior and other parameters are used. Furthermore, the intersection designs studied in this research are not being tested based on a particular location. These designs are rather tested to represent various field conditions, mainly balanced and unbalanced conditions, peak and off peak conditions, etc. Calibration is accounted for by testing the different flow scenarios presented in the previous chapter with different left and right turning percentages to ensure that the intersections can be applied for all field conditions.
4.4 Unsignalized Roundabout Network
The VISSIM unsignalized roundabout model was obtained from Dr. Praveen K.
Edara, assistant professor at the Department of Civil and Environmental Engineering of the University of Missouri, Columbia. The roundabout modeled in this research
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contained two through lanes. The geometric elements of the roundabout are illustrated in
Table 3 1. The geometry of the roundabout was adjusted in order to match the geometry of the other intersection types. After the necessary changes were made, the model was tested to ensure that it worked properly. As previously mentioned in section 3 1, lane widths were specified as 12 feet and no gradient was used. Desired speeds on the approach links were set to be 50 km/hr (approximately equivalent to 30 mph) for cars and
40 km/hr (25 mph) for trucks. To ensure safe operation, the desired speeds on the entries, circulating, and exiting curves were set as 25 km/hr (15 mph) and 20 km/hr (12 mph) for cars and trucks respectively. Two percent of the total entering flow was assumed as percentage of trucks on the network for the model. The priority rules in VISSIM, which check for minimum gap time and minimum headway, were ensured to be properly placed in order to avoid conflicts between vehicles on the network. Priority is given to vehicles already circulating in the roundabout.
To generate the same model in AIMSUN, the built in import feature in AIMSUN was used. The VISSIM model of the roundabout was simply imported into AIMSUN which made it easier to generate the model. However, since VISSIM uses a links and connectors system to draw the network, different from AIMSUN which uses nodes and links system, a considerable amount of time was spent to make certain that the links were properly connected. Once the model was properly drawn, the same parameters used in
VISSIM was used in the AIMSUN model in order to conduct a fair comparison. Since one of the objectives of this research is to compare the simulation platforms’ performance. Default parameters were used for the model logic and driver behavior.
Screenshots of the AIMSUN and VISSIM roundabout models are shon in figure 4 1.
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Figure 4 1: Screenshots of the Roundabout Models in AIMSUN (Left) and VISSIM (Right)
4.5 Signalized Roundabout Network
To generate the signalized networks in both AIMSUN and VISSIM, the signal timing plans presented in section 3 3 were implemented in each of the models. No changes were made to geometry, and other inputs of the unsignalized models to maintain consistency.
4.6 Continuous Flow Intersection Network
The VISSIM continuous flow intersection model was also obtained from Dr. Edara.
The same procedure used to alter the geometry of the roundabout was followed for the continuous flow intersection model in VISSIM. Approach speed for cars and trucks, percentage trucks, lane widths, and gradients were specified as in the roundabout model.
Differently from the roundabout, however, the continuous flow intersection contained left turning and right turning bays as well as acceleration lanes. Refer to section 3 1 for a detailed geometry of the continuous flow intersection modeled in this research. The
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signal control settings, as seen in figure 3 6, were implemented into the model for each of the various flows.
The model was then imported into AIMSUN, and the necessary coding was undertaken to ensure that the models are not only comparable within the simulation platforms but also to the other intersection types. Figure 4 2, below, shows a screenshot of the AIMSUN (left) and VISSIM model of the continuous flow intersection.
Figure 4 2: Screenshots of the Continuous Flow Intersection Models in AIMSUN (Left) and VISSIM
(Right)
4.7 Parallel Flow Intersection Network
To generate the parallel flow intersection network in VISSIM, an AutoCAD layout of the intersection was imported into the software and set as background. The CAD background was scaled to match the interface of the VISSIM platform to ensure that all the measurements are in the same units. Once the background is properly scaled, the
VISSIM links and connectors were drawn. The desired input data such as routes, lane assignments and geometries, traffic demands, and turning percentages by different vehicle types, and signal control settings were inputed in the model. As previously 83
stated, parameters such as desired speeds, turning percentages, gradients ect. are set to be the same in all the intersection models. The AIMSUN model was generated as in the case of the roundabout and the continuous flow intersection. Figure 4 3 presents screenshots of the parallel flow intersection models in AIMSUN and VISSIM.
Figure 4 3: Screenshots of the Continuous Flow Intersection Models in AIMSUN (Left) and VISSIM
(Right)
4.8 Conventional Intersection Network
The conventional intersection model was created in VISSIM and AIMSUN according to the same procedure employed to create the parallel flow intersection models.
The model was first created in VISSIM by drawing the links and connectors on the scaled
AutoCAD background and ensuring proper coding of the network and that all parameters are kept constant, similar to the other intersection networks to ensure a fair comparison. 84
Then the model was imported into AIMSUN and properly coded as was done for the other intersection designs. Screenshots of the conventional intersection models are presented in figure 4 4.
Figure 4 4: Screenshots of the Conventional Intersection Models in AIMSUN (Left) and VISSIM
(Right)
4.9 Traffic Volume Data
The traffic volume data used for this study are presented in the following tables. As previously stated, three major issues were considered in generating these volumes: simulating peak and off peak conditions, accounting for balanced and unbalanced conditions, and quantifying the impact of left turn movement to the intersections. The different volumes generated for the analysis can be seen in Tables 4 1 through 4 5.
Tables 4 1, 4 2, and 4 3 present the balanced flow condition as well as the left turning percentages considered in this research; with 15%, left turning presented in Table
4 1, 20% and 25% in Tables 4 2 and 4 3 respectively.
Table 4 1: Uniformly Balanced Flow with 20% Left Turn
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Traffic Eastbound Westbound Northbound Southbound
Volumes (veh/hr) (veh/hr) (veh/hr) (veh/hr)
(veh/hr) L T R L T R L T R L T R
1000 38 188 25 38 188 25 38 188 25 38 188 25
1500 56 281 38 56 281 38 56 281 38 56 281 38
2000 75 375 50 75 375 50 75 375 50 75 375 50
2500 94 469 63 94 469 63 94 469 63 94 469 63
3000 113 563 75 113 563 75 113 563 75 113 563 75
3500 131 656 88 131 656 88 131 656 88 131 656 88
4000 150 750 100 150 750 100 150 750 100 150 750 100
4500 169 844 113 169 844 113 169 844 113 169 844 113
5000 188 938 125 188 938 125 188 938 125 188 938 125
6000 225 1125 150 225 1125 150 225 1125 150 225 1125 150
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Table 4 2: Uniformly Balanced Flow with 20% Left Turn
Traffic Eastbound Westbound Northbound Southbound
Volumes (veh/hr) (veh/hr) (veh/hr) (veh/hr)
(veh/hr) L T R L T R L T R L T R
1000 50 175 25 50 175 25 50 175 25 50 175 25
1500 75 263 38 75 263 38 75 263 38 75 263 38
2000 100 350 50 100 350 50 100 350 50 100 350 50
2500 125 438 63 125 438 63 125 438 63 125 438 63
3000 150 525 75 150 525 75 150 525 75 150 525 75
3500 175 613 88 175 613 88 175 613 88 175 613 88
4000 200 700 100 200 700 100 200 700 100 200 700 100
4500 225 788 113 225 788 113 225 788 113 225 788 113
5000 250 875 125 250 875 125 250 875 125 250 875 125
6000 300 1050 150 300 1050 150 300 1050 150 300 1050 150
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Table 4 3: Uniformly Balanced Flow with 25% Left Turn
Traffic Eastbound Westbound Northbound Southbound
Volumes (veh/hr) (veh/hr) (veh/hr) (veh/hr)
(veh/hr) L T R L T R L T R L T R
1000 63 163 25 63 163 25 63 163 25 63 163 25
1500 94 244 38 94 244 38 94 244 38 94 244 38
2000 125 325 50 125 325 50 125 325 50 125 325 50
2500 156 406 63 156 406 63 156 406 63 156 406 63
3000 188 488 75 188 488 75 188 488 75 188 488 75
3500 219 569 88 219 569 88 219 569 88 219 569 88
4000 250 650 100 250 650 100 250 650 100 250 650 100
4500 281 731 113 281 731 113 281 731 113 281 731 113
5000 313 813 125 313 813 125 313 813 125 313 813 125
6000 375 975 150 375 975 150 375 975 150 375 975 150
Tables 4 4 and 4 5 illustrate the unbalanced flow conditions. The volumes shown in
Table 4 4 were divided considering a major road (East West) with 70% of the total flow and a minor road (North South) with the remaining 30%. To create the unbalanced scenario, the flows on the major road were distributed with 60% assigned to the eastbound approach and 40% assigned to the westbound approach. The flows on the minor road were distributed evenly as seen in the table. Table 4 5 presents the unbalanced flow scenario obtained from Bared and Edara (2005).
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Table 4 4: Unbalanced Flow Condition Considering a Major and a Minor Road
Traffic Eastbound Westbound Northbound Southbound
Volumes (veh/hr) (veh/hr) (veh/hr) (veh/hr)
(veh/hr) L T R L T R L T R L T R
1000 63 315 42 42 210 28 38 98 15 38 98 15
2000 126 630 84 84 420 56 75 195 30 75 195 30
3000 189 945 126 126 630 84 113 293 45 113 293 45
4000 126 630 84 84 420 56 150 390 60 150 390 60
5000 315 1575 210 210 1050 140 188 488 75 188 488 75
Table 4 5: Unbalanced Flow Condition Adopted from Bared and Edara (2005)
Traffic Eastbound Westbound Northbound Southbound
Volumes (veh/hr) (veh/hr) (veh/hr) (veh/hr)
(veh/hr) L T R L T R L T R L T R
1825 75 425 50 50 325 75 75 300 75 50 250 75
2740 110 628 88 126 474 75 113 450 113 75 375 113
3650 150 850 100 150 650 100 150 600 150 100 500 150
4.10 Signal Timing Plans
The optimum cycle lengths and green splits obtained from PASSER II for the volume scenarios considered are presented in the following tables. These signal settings were used for the conventional intersection design and the signalized roundabout. Table
3 9 shows the optimized signal control plans for the balanced volumes while the control plans for the unbalanced volumes are illustrated in table 3 10.
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Table 4 6: Optimized Signal Timing Plans for Conventional Intersection and Signalized Roundabout
for Balanced Flow Scenarios
Green Splits (sec)
Total Traffic Optimized
Volumes Cycle Length
(veh/hr) (sec)
1000 60 10 20 10 20
1500 60 10 20 10 20
2000 60 10 20 10 20
2500 60 10 20 10 20
3000 60 10 20 10 20
3500 60 10 20 10 20
4000 70 12 23 12 23
4500 70 12 23 12 23
5000 90 15 30 15 30
6000 120 19 41 19 41
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Table 4 7: Optimized Signal Timing Plans for Conventional Intersection and Signalized Roundabout
for Unbalanced Flow Scenarios
Green Splits (sec)
Total Traffic Optimized
Volumes Cycle Length
(veh/hr) (sec)
1000 60 10 25 10 15
1825 60 12 21 10 17
2000 60 10 25 10 15
2740 60 12 22 10 16
3000 60 16 19 10 15
3650 60 12 22 10 16
4000 80 21 31 13 15
5000 120 32 48 18 22
The signal timings for the continuous flow and the parallel flow intersections were derived following the pre timed control setting shown in figure 3 6. The green interval used for the intersections was 30 s for Phase 1 and 2, and the corresponding yellow time and all red intervals were 4 + 2 s. The green intervals were 43 s and 17 s for Phases 3, 4,
5, and 6, respectively. The yellow time and all red intervals are 4 + 2 s for all these movements. The total cycle length for all scenarios was 72 s. Overall, the signals for both the continuous flow and the parallel flow intersection designs operate under two phases.
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4.11 Simulation Scenarios
The variation of traffic demand volumes generated a total of 38 scenarios for each intersection types. Once all models were created and were properly examined, the following scenarios were followed for each of the simulation:
• Simulation runs for both AIMSUN and VISSIM models were
performed on the same computer to ensure consistency because the
simulation can be affected by the type of computer used, the memory and the
processor of the computer (Xeon ™ CPU 3.20 GHZ, 2.00 GB of RAM)
• Simulation time for each run was 4,800 seconds and the warm up
period was set to 1,200 seconds for each model and repeated for 7,200
seconds to examine whether the run time had an effect on the results
• Ten replications were performed for each simulation to ensure that
there were no significant discrepancies in the result. Statistical analysis of
data from a pilot study indicated that 10 simulation runs were sufficient to
account for the random variation in driving behavior in each scenario
(Rayaprolu, 2009). The number of seeds used affects the results of a
simulation run. For instance, if ten replications are performed with the same
number of random seeds, the ten replications will yield the same results.
Therefore, different seed numbers were used for each of the ten simulation
runs. Different random seed values are assigned to each replication in
AIMSUN by default. In VISSIM, the multirun feature is used to select 10
replication and assign different starting and incremental seed numbers. In
case an inconsistency was observed in the preliminary results between the
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different replications, the entire animated simulation run was observed in
order to detect the problem and fix it. An average of the replications was
then computed.
• Once everything was observed to be properly working, the necessary
measures of effectiveness were extracted from the microsimulation platforms
for statistical analysis and comparison between the intersection designs.
4.12 Measures of Effectiveness
The measures of effectiveness used to evaluate and compare the performance of the intersections in this research were selected considering several factors. The first, perhaps most significant factor taken into consideration the relevance of the particular performance measures in the field for each type of intersections given that some are signalized and the roundabout is not. In addition, since results are being compared using two different microsimulation platforms, the availability of the performance measures from the models used must be comparable.
Delay is a standard parameter used to measure the performance of an intersection.
The Highway Capacity Manual identifies delay as the primary measure of effectiveness for both signalized and unsignalized intersections, with level of service determined from the delay estimate (Transportation Research Board, 2000). Zhou et al., (2002) evaluated several unconventional intersections and considered average delay time and travel time as measures of effectiveness. The authors concluded that average delay is a critical performance measure of operations on interrupted flow facilities and clearly reflects greater discomfort caused to drivers than travel time. Therefore, average delay time is
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used as the primary measure of effectiveness to evaluate the performance of the intersection designs.
In addition to average delay, several other factors can be taken as measures of effectiveness. The Federal Highway Administration’s roundabout guide suggests queue length as one of the key measure of effectiveness when assessing the adequacy of the geometric design of a roundabout. Average queue length, which is equivalent to the vehicle hours of delay per hour on an approach, is useful for comparing roundabout performance with other intersection forms (Robinson et al., 2000). Jagannathan and
Bared (2004) used average queue length as a performance measure in their evaluation of different variations of a continuous flow intersection using VISSIM as a simulation tool to produce the desired measures of effectiveness. Number of stops can also be considered as a measure of effectiveness. Since several unconventional intersections are being evaluated, the number of stops as a performance measure would clarify the number of time, on average, a driver has to stop while travelling through each type of intersection.
Average delay and average queue length are available in both Aimsun 6.0 and
VISSIM 5.10 as measures of effectiveness. VISSIM defines average total delay, in seconds per vehicle, as the difference between the desired travel time and actual travel time. AIMSUN defines average delay time, in seconds per vehicle, as the difference between the expected travel time (time it would take to traverse the section under ideal condition) and the travel time. Number of stops in AIMSUN is defined as the average number of stops per vehicle while travelling in the section. VISSIM’s performance measure, stops, is the average number of stops per vehicle travelling through the
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intersection. Whereas average delay time and number of stops may be compared between VISSIM and AIMSUN, average queue is not comparable between the two platforms. VISSIM defines average queue length as the average queue length in the network in length units while, in AIMSUN, average queue length is expressed as a number of vehicles per lane in a section.
Based on the criteria presented in this section, average delay and number of stops are the two measures of effectiveness selected to evaluate and compare the operational performance of each intersection design. The performance measures obtained from the models are used as the basis for the comparison between the intersections. The results are analyzed in the following section and conclusions are drawn from the analysis.
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5 RESULTS AND ANALYSIS
Using the two microsimulation platforms: AIMSUN and VISSIM, thirty eight scenarios were tested for each of the intersections in this research with regards to balanced condition , unbalanced condition, and the effect of left turning movement. Ten scenarios were considered for the balanced flow condition while eight volume scenarios were run for the unbalanced condition. The effect of left turn movement of the intersections was quantified by testing 15%, 20%, and 25% left turning for the balanced scenarios. These left turning percentages represent different field observations where
15% is typical in most cases and 25% represents a high left turn movement. Ten replications with different seed numbers were used and averaged to ensure the accuracy of the results. Two primary measures of effectiveness: average delay time and number of stops, were used to assess the operational performance of the intersections.
Conclusions are drawn from the results obtained from the two microscopic simulation platforms. In addition, a sensitivity analysis for the average delay times obtained is conducted to determine which unconventional intersection design is an appropriate alternative to the conventional intersection for low volume level (1,000 – 2.500 vph), medium volume level (3,000 – 4,000 vph), and high volume level (4,500 – 6,000 vph).
The results simultaneously compare the intersections and the two simulation platforms in order to establish the similarities and differences between the two models. Finally, statistical analysis is conducted to compare results obtained from the two models.
Furthermore, a general comparison of the two models with regards to network coding 96
process, simulation processing time, and data output is provided to emphasize some of the key differences between the models.
5.1 Balanced Traffic Scenarios
For the balanced flow condition, ten volumes were run from a total entering volume of 1,000 vehicles per hour to 6,000 vehicles per hour with a 500 vehicles per hour increment. These volumes were generated hypothetically to emulate peak and off peak traffic data that can be observed in the field where 1,000 vehicles per hour represent a low entering volume and conversely 6,000 vehicles per hour represent a high entering volume at a typical intersection. The purpose of the 500 vehicles per hour increment was to help account for the minimal changes in the intersection performance between the off peak and peak conditions. The results for the balanced flow scenarios are presented in Tables 5 1, which illustrates the average delay time in seconds per vehicle (s/veh) for the intersections for the 15% left turn movement, establishing the comparison between the alternatives. The comparison between the microsimulation platforms AIMSUN and
VISSIM are also illustrated in the tables for each individual intersection design.
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Table 5 1: Average Delay Time Comparison for 15% Left Turn Movement for the Balance Flow Scenario
Total Avg. Delay Time (s/veh)
Entering Signalized
Volume Conventional Roundabout Roundabout Continuous Flow Parallel Flow
(vph) AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM
1000 18.9916 18.3143 5.4829 4.8944 9.0054 8.6624 17.1925 19.6586 14.5077 14.1817
1500 18.5131 19.2606 7.1040 6.4667 10.0773 9.8183 19.5929 20.3471 16.0321 16.1634
2000 19.8403 20.0183 9.2949 9.1200 11.5152 10.8029 20.3249 20.4571 18.9346 19.4851
2500 20.0357 20.4306 13.0433 12.8409 14.2252 13.1755 20.7434 20.6714 23.4491 23.1266
98 3000 21.8560 21.9117 38.8082 35.9734 19.4329 18.8447 21.0068 22.0114 24.1452 24.2919
3500 22.5803 23.3771 86.3979 78.6220 28.0375 27.9116 21.7041 22.7343 27.2713 27.6011
4000 27.1996 29.6814 148.5090 121.4379 41.6214 40.2451 22.1102 22.8686 28.1762 28.0628
4500 28.1327 31.3157 195.5810 186.5419 51.8016 49.0532 25.5034 23.7943 31.0384 30.4862
5000 35.9758 38.4814 329.2110 300.0991 85.3239 84.5410 27.0370 24.0471 32.0749 33.0016
6000 53.0994 61.7214 358.1620 314.3803 200.3100 198.3373 26.7730 25.6981 37.4633 37.9336
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For the 15% left turn movement, the average delay time for the conventional intersection gradually increases by an average of approximately one second per every 500 vehicles increment from a total entering volume of 1,000 vehicles per hour to 3,500 vehicles per hour. As the total entering volume increases for the intersections, the average delay per vehicle drastically increases. At the highest tested volume (6,000 vph), an average delay of approximately 53 seconds is experienced per vehicle. This is consistent with the hypothesis that the conventional intersection does not perform well at volume level close to saturation. According to the criteria established by the Highway
Capacity Manual (HCM) 2000 (Table 2 3), from 1,000 to 3,500 vehicles per hour, the conventional intersection performed at a level service (LOS) C. While a level of service
(LOS) D was achieved from 4,000 to 4,500 vph, the unconventional intersection performed at LOS F for flows higher than 5,000 vehicles per hour.
Vehicles in the un signalized roundabout experienced the lowest delay compared to the other intersections evaluated in this research at the low volume levels with average delay time of less than 10 seconds per vehicle up to 2,000 vehicles per hour. According to the criteria established by the HCM 2000 (Table 2 2), the un signalized roundabout performed at a level of service (LOS) A for entering volumes up to 2,500 vehicles per hour. However, once the total entering volume exceeds 3,000 vehicles per hour, the average delay increase for the roundabout is extreme. As seen in Table 5 1, at 6,000 vehicles per hour, the average delay time per vehicle was more than 300 seconds. This phenomenon can be explained by the fact that vehicles entering the roundabout have to stop and wait until a gap exists since priority is given to the vehicles circulating the roundabout. As the volume increases on the approaches, the capacity of the roundabout
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is rapidly exceeded, a fact, which makes it unable to suitably process the total entering, flow and results in extreme delay. At 3,000 vehicles per hour, the level of service of the un signalized roundabout was LOS E and from 3,500 to 6,000 vph the intersection performed at LOS F.
The results show that introducing signals at the roundabout is beneficial and effective especially at higher volume level when the roundabout capacity is exceeded.
Up to 2,500 entering vehicles per hour, both the un signalized roundabout and the signalized roundabout performed at a LOS A, although the un signalized roundabout produced slightly lower delays at these volume levels. However, beyond 2,500 vehicles per hour, introducing the signals significantly reduces the average delay time at the roundabout. For instance, it can be observed that at 5,000 vehicles per hour, the average delay at the roundabout decreased from approximately 300 seconds to 80 seconds.
Although the signals proved to be beneficial in terms of lessening the delay experienced at the roundabout, it should be noticed that the delay at the higher volume levels are higher than the conventional intersection. It can therefore be concluded that the double lane roundabout is not a feasible alternative to the conventional intersection for entering flow of more than 3,000 vehicles per hour.
Compared to the conventional intersection, the parallel flow produced better performance results in terms of lower delays for most of the cases in the balanced volume scenarios. For the 1,000 entering vehicles per hour scenario, the parallel flow intersection resulted in a LOS B while the conventional intersection had a LOS C. At entering volume of up to 4,500 vehicles per hour, the average delay times per vehicle produced by the parallel flow intersection were similar to the conventional intersection,
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both resulting in LOS C or D for the medium volume levels. The operational benefit of the parallel flow intersection, when compared to the conventional intersection, is amplified at the highest tested flow (6,000 vph) with a reduction of approximately 20 seconds per vehicle.
The continuous flow intersection, on the other hand, had the best overall performance for the balanced scenarios with 15% left turn movement compared to the other intersection designs evaluated in this research. At the low entering flow levels (1,000 – 2,500 vph), the continuous flow intersection produced average delay times that are similar to the conventional intersection. However, at the high flow levels, the continuous flow intersection produced lower average delays than all the other alternatives with the conventional intersection and the roundabout performing at a LOS F, the parallel flow intersection at LOS E while the continuous flow intersection had a LOS C. As shown in
Table 5 1, the difference in average delay time at 6,000 entering vehicle per hour between the continuous flow intersection and the conventional intersection was more than 26 seconds for the balanced scenario.
Figures 5 1 and 5 2 below graphically illustrate the delay time results of each intersection designs for 15% left turn volume. Figure 22 shows the average delay times obtained from AIMSUN while the results for the intersections from VISSIM are shown in Figure 23.
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Conventional AIMSUN Avg DelayTime Comparison for 15% Left Turn Roundabout
400.0000 Signalized Roundabout 350.0000 CFI
300.0000 PFI 250.0000
200.0000
150.0000
100.0000 Avg. Delay TimeAvg. Delay (s/veh) 50.0000
0.0000 1000 2000 3000 4000 5000 6000 Total Enetring Volume (vph)
Figure 5 1: Average Delay Time Comparison for 15% Left Turn Movement for Results Obtained
from AIMSUN 6.0
Conventional VISSIM Avg DelayTime Comparison for 15% Left Turn Roundabout
400.0000 Signalized Roundabout 350.0000 CFI
300.0000 PFI 250.0000
200.0000
150.0000
100.0000 Avg. Delay TimeAvg. Delay (s/veh) 50.0000
0.0000 1000 2000 3000 4000 5000 6000 Total Enetring Volume (vph)
Figure 5 2: Average Delay Time Comparison for 15% Left Turn Movement for Results Obtained
from VISSIM 5.10
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Tables 5 2 presents the average number of stops in unit of number per vehicle per mile (#/veh/mi) for the studied intersections for 15% left turn movement. It was observed that the average number of stops per vehicle follow a similar trend as the average delay time. This is due to the fact that, as more vehicles approaches the intersections the level of congestion increases. The queue formed on the intersection approaches lengthen and vehicles experience more stops per cycle length before reaching the main intersection.
At low entering volume (1,000 – 2,500 vph), vehicles at the un signalized roundabout stop less frequently compared to the other intersections. This is consistent with the lower average delay experienced by the vehicles as shown in Table 5 1.
Conversely, as the delay increases in the roundabout, so does the number of stops experienced by vehicles travelling through the intersection. For the higher flow level, vehicles stop approximately four times per mile before leaving the intersection. The parallel flow intersection and the continuous flow intersections had a higher average number of stops compared to the conventional intersections for most of the entering flows. This trend occurs because vehicles have to travel longer distances before leaving the intersection due to displaced left turn bays and therefore have to stop more frequently to wait while conflicting vehicles have priority during their respective green phases.
However, the difference in number of stops per mile experienced at the unconventional intersections does not differ much from the conventional intersection.
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Table 5 2: Average Number of Stops Comparison for 15% Left Turn Movement for the Balance Flow Scenario
Number of Stops (#/veh/mi)
Total Signalized
Entering Conventional Roundabout Roundabout Continuous Flow Parallel Flow
Volume (vph) AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM
1000 0.7516 0.7199 0.1259 0.0600 0.4305 0.2136 0.7380 0.8669 0.8008 0.8413
1500 0.7615 0.7332 0.2327 0.1884 0.5053 0.4561 0.8425 0.8777 0.8616 0.8735
2000 0.7822 0.7416 0.3582 0.3888 0.5925 0.6002 0.8782 0.8837 0.8864 0.8961
2500 0.7834 0.7588 0.5538 0.5234 0.7807 0.7564 0.9139 0.8971 0.9477 0.9709 104 3000 0.7851 0.7609 0.9559 0.9467 1.1696 0.9375 0.8475 0.9343 0.9644 0.9813
3500 0.8026 0.7658 1.9626 1.8212 1.3248 1.0866 0.8746 0.9409 0.9711 0.9886
4000 0.8452 0.8154 2.7139 2.0067 1.5840 1.3015 0.9153 0.9526 0.9554 0.9970
4500 0.8672 0.8316 3.2603 2.9149 2.0156 1.7681 1.0721 0.9639 1.0017 1.2116
5000 0.9111 0.8821 3.3827 3.1175 2.2005 2.0096 1.1392 0.9701 1.2083 1.4164
6000 1.6002 1.2654 4.1108 3.8824 3.6552 3.2291 1.1047 1.0003 1.3442 1.6638
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As the left turn percentage was increased in the balanced scenarios in order to quantify the left turn movement on the each intersection, the average delay time and the number of stops at the intersections also increased in the same manner. Tables 7 1 to 7 4 illustrate the variation in delay time and number of stops at the intersection for the 20% and 25% left turn volumes (See Appendix). Figures 5 3 and 5 4 graphically compare the average number of stops at the intersection. As seen in the figures, the un signalized roundabout had the lowest average number of stops for total entering volumes of up to
3,000 vehicles per hour. It can also be observed that adding signal to the roundabout reduced the number of stops at the un signalized roundabout for the higher entering volumes. Vehicles at the continuous flow intersection and the parallel flow intersection experienced similar average number of stops as the conventional intersection although the conventional intersection had slightly lower number of stops than the other two.
Conventional AIMSUN Avg Number of Stops Comparison for 15% Left Turn Movement Roundabout 5.0000 Signalized 4.5000 Roundabout CFI 4.0000
3.5000 PFI
3.0000
2.5000
2.0000
1.5000
1.0000
Average Number of Stops (#/veh/mi) Stops of Number Average 0.5000
0.0000 0 1000 2000 3000 4000 5000 6000 Total Entering Flow (vph)
Figure 5 3: Average Number of Stops Comparison for 15% Left Turn Movement for Results
Obtained from AIMSUN 6.0
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Conventional VISSIM Average Number of Stops Comparision for 15% Left Turn Volume Roundabout
4.5000 Signalized Roundabout 4.0000 CFI
3.5000 PFI
3.0000
2.5000
2.0000
1.5000
Number of Stops (#/veh/mi) Stops of Number 1.0000
0.5000
0.0000 1000 2000 3000 4000 5000 6000 Total Entering Volume (vph)
Figure 5 4: Average Number of Stops Comparison for 15% Left Turn Movement for Results
Obtained from VISSIM 5.10
5.2 Unbalanced Traffic Scenarios
For the unbalanced condition, eight scenarios were considered: five scenarios with total entering volumes from 1,000 to 5,000 vehicles per hour with 70% of the volume attributed to the major road (East – West) and 30% of the volume to the minor road
(North – South), and three random scenarios. Fifteen percent left turn percentage and ten percent right turn was used for all the scenarios. Table 5 3 summarizes the average delay times for the intersections with respect to the unbalanced scenarios.
For the low traffic volume levels (1,000 vph – 2,000 vph) the un signalized roundabout showed smaller average delays than the other alternatives. However, as the volume was increased, the average delay for the un signalized roundabout rapidly
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increased. The signalized roundabout performed better than the un signalized roundabout at medium to high volume levels in terms of lower average delay; but the average delays for the signalized roundabout were considerably higher than the other intersections. At traffic volumes higher than 2,000 vehicles per hour, the continuous flow intersection showed lower average delay than the conventional intersection as well as the other unconventional intersection designs. The parallel flow intersection showed similar average delays as the continuous flow intersection at low to medium volumes. The difference between the two unconventional intersections was established at the total approach volume of 5,000 vph where the parallel flow intersection had an average delay of 37 seconds among the two simulation platforms while the continuous flow produced an average delay of approximately 28 seconds. Although the continuous flow intersection produced better result than the parallel flow intersection, it should be noted that the parallel flow intersection showed lower delay than the conventional intersection for most of the volume levels.
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Table 5 3: Average Delay Time Comparison for the Unbalanced Scenarios
Total Avg. Delay Time (s/veh)
Entering Signalized
Volume Conventional Roundabout Roundabout Continuous Flow Parallel Flow
(vph) AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM
1000 18.0078 17.9863 5.1822 3.0364 9.0054 8.4373 17.3548 19.5773 20.0238 18.3319
1825 20.4537 20.5312 7.8594 6.1942 9.2797 8.8905 18.4095 20.5300 20.6514 19.6421
2000 22.3996 22.3108 8.8023 7.0069 11.5152 9.8321 19.8227 20.5731 21.3622 20.3900
2740 24.8420 24.7339 24.5611 23.9908 19.7386 16.4164 21.5970 21.3037 22.3510 22.4365 108 3000 27.6725 28.0065 30.1614 31.4763 30.2717 27.7622 21.6608 21.8207 24.6354 23.9048
3650 31.9864 32.2210 41.7137 42.0612 33.1335 31.2571 21.9611 22.3336 26.3461 24.3413
4000 42.1917 40.9036 230.3790 189.7441 85.6877 78.1743 24.2494 23.1686 31.7231 29.4886
5000 91.7987 83.1744 303.5040 246.0719 128.5930 106.6632 31.8939 24.5086 38.0078 36.7705
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5.3 Analysis of Left Turn Movement
The effect of left turn movement of each of the unconventional intersections was quantified by testing three distinct left turn percentages: 15%, 20%, and 25%. Figures 5
5, 5 6, and 5 7 illustrate the changes in average delay time for the roundabout, the parallel flow intersection and the continuous flow intersection respectively. As seen in
Figure 5 5, the average delay time for the roundabout increases as the total approach volume increases. At the lower volumes (1,000 – 2,500 vph) the average delay time for the roundabout was generally unchanged. As the volume increases, the effect of the left turn percentage is gradually seen with the 25% left turn producing a higher delay to the overall intersection.
Roundabout Avg. Delay Time
450
400
350
300
250 15% Left Turn (s/veh) 20% Left Turn (s/veh) 200 25% Left Turn (s/veh)
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Avg. Delay TimeAvg. Delay (s/veh) 100
50
0 0 1000 2000 3000 4000 5000 6000 7000 Total Entering Flow (vph)
Figure 5 5: Average Delay Time Comparison for Roundabout for Different Left Turn Movement
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The parallel flow intersection is similar to the roundabout in that the increase in left turn percentage affects the intersection in a similar way. Figure 5 6 shows that the increase in left turn percentage marginally affects the parallel flow intersection. As the left turn percentage increases the parallel flow intersection shows slightly higher delay.
The continuous flow intersection, on the other hand, exerts a different behavior with respect to the increase in left turn percentage. From 1,000 to 4,500 total entering vehicles, the three different left turn percentages tested show similar average delays.
However, as the total volume increase, the 25% left turn resulted in a lower delay while the 15% and 20% left turn produce practically the same average delays. The additional left turn bays provided in the continuous flow intersection design are able to accommodate the increase in left turn vehicles and therefore reducing the impact of these vehicles on the overall intersection. Removing the left turn from the intersection and crossing the incoming traffic before reaching the main junction is proven to be beneficial to the entire intersection.
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PFI Avg. Delay Time
45.0000
40.0000
35.0000 15% Left Turn (s/veh/mi) 30.0000 20% Left Turn (s/veh/mi)
25.0000 25% Left Turn (s/veh/mi)
20.0000
15.0000
Avg. Delay TimeAvg. Delay (s/veh) 10.0000
5.0000
0.0000 0 1000 2000 3000 4000 5000 6000 7000 Total Entering Volume (vph)
Figure 5 6: Average Delay Time Comparison for Parallel Flow Intersection for Different Left Turn
Movement
CFI Avg. Delay Time
30
25
20 15% Left Turn (s/veh/mi) 20% Left Turn (s/veh/mi) 15 25% Left Turn (s/veh/mi)
10 Avg. Delay Time Avg. Delay (s/veh) 5
0 0 1000 2000 3000 4000 5000 6000 7000 Total Entering Volume (vph)
Figure 5 7: Average Delay Time Comparison for Continuous Flow Intersection for Different Left
Turn Movement
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5.4 Average Delay Time Sensitivity Analysis
To determine the appropriate alternative for each of the volume level considered: low (1,000 – 2,500 vph), medium (3,000 – 4,000 vph), and high (4,500 – 6,000 vph), a sensitivity analysis was conducted to compare the intersections. There are two major classes of sensitivity functions: analytic and empirical. Analytic sensitivity functions are used for well defined systems usually using partial derivatives whereas empirical sensitivity functions show sensitivity to parameters by observing system changes when certain parameters are changed. The latter are used for un modeled systems (Bahill et al.,
2008).
Bloomberg and Dale (2000) performed a sensitivity analysis to compare the relative travel times for seven different routes on a congested network as predicted by two microsimulation models: CORSIM and VISSIM. After calibrating and validating the models and ensuring that the models produce results that are comparable to field observations, results for two sets of runs were obtained where one set used traffic demands 10% lower than the base case and the other used 10% higher than the base case.
This sensitivity analysis established whether the relative comparisons between the scenarios were consistent even with higher or lower traffic demands. Then, a system was developed to compare the travel times for the seven alternatives where the system travel time values were scaled to 100. For instance, if the fastest travel time was 30 minutes, that scenario would be given a score of 100. If another scenario then had a travel time of 40 minutes, that scenario would be given a relative score of 100* 30/40 =
75. Finally, the alternative with the highest score is considered the best alternative.
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A similar method is appropriate to compare the alternative in this research and obtain the appropriate alternative to the conventional intersection for each aforementioned volume level as well as the best overall alternative. The sensitivity analysis was conducted for the balanced volume scenarios since approximately the same trend was observed for the most of the scenarios. For each of the entering volumes, the alternative with the lowest average delay time is awarded a score of 100 then each of the other alternatives is awarded a relative score as described above. A total score is determined for each of the flow level separately so that the best alternative for each level can be determined. Then, These values are added in order to determine the overall best alternative between the intersection designs. Table 5 4 presents the result of the sensitivity analysis for the results obtained from AIMSUN while Table 5 5 shows the sensitivity analysis result for the values obtained from VISSIM.
Both the AIMSUN and VISSIM sensitivity analysis results are consistent in suggesting that for the low volume level (1,000 – 2,500 vph), the roundabout is the best alternative while the continuous flow is the best alternative for the medium (3,000 –
4,000 vph) and high (4,500 – 6,000 vph) volumes levels. Moreover, the continuous flow is the best overall alternative compared to the other intersections.
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Table 5 4: Sensitivity Analysis Comparison for AIMSUN Results 114
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Table 5 5: Sensitivity Analysis Comparison for VISSIM Results 115
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5.5 Comparison of AIMSUN and VISSIM
As illustrated in figures 5 1 and 5 2 as well as Tables 5 1 to 5 3, the results obtained from AIMSUN were similar to those obtained from VISSIM. Figure 5 8 below exemplified the difference between the average delay times obtained from AIMSUN and
VISSIM for the continuous flow intersection considering 15% left turn movement.
Average Delay Time for Continuos Flow Intersection for 15% Left Turn Movement
30.0000
25.0000
20.0000
AIMSUN 15.0000 VISSIM
10.0000 Avg. Delay Time Avg. Delay (s/veh) 5.0000
0.0000 1000 2000 3000 4000 5000 6000 Total Enterring Volume (vph)
Figure 5 8: AIMSUN vs. VISSIM Average Delay Time for Continuous Flow Intersection for 15%
Left Turn Movement
To establish whether the differences between the results from the two microsimulation platforms were significant, statistical analyses were conducted. A paired t test was applied to compare the results from the two simulation platforms. This statistical test uses independent samples to make inferences about population means. For the purpose of this research, values obtained from the two models for one of the intersections are selected for the statistical analysis. The t test is therefore designed in
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this case to test whether the two microscopic platforms report significantly different average delay time values for the same intersection.
A 95% confidence level (α = 0.05) is chosen, which means that for 95 independent trials out of 100, repeated sampling will have the same mean value µ. The average delay time values obtained from the two platforms for the continuous flow intersection for 15% left turn movement is selected for the statistical analysis. The t test is conducted using the following equations adopted from Ott and Longnecker (2001).
… … … (5 1)
… … … (5 2)
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Where,
µ 1: The mean of population 1
µ 2: The mean of population 2
D o: Specified value, often taken to be 0
Sp: Estimate for the common standard deviation
n 1: Number of samples taken from population 1
n 2: Number of sample taken from population 2
S 1: Standard deviation of sample 1
S 2: Standard deviation of sample 2
t: The test statistic
Ŷ 1: Mean value of sample 1
Ŷ2: Mean value of sample 2
t α / 2: Test statistic threshold for confidence level α
From the sample quantities presented in Table 5 1 for the continuous flow intersection, the following information can be determined:
AIMSUN VISSIM n 1 = 10 n 2 = 10 Sp = 2.673168
Ŷ 1 = 22.1998 Ŷ 2 = 22.2288 | t |= 0.025085
S 1 = 3.238806 S 2 = 1.94982 t α / 2 = 2.262
Since t paired = 0.025085 is less than t α / 2 = 2.262, we can conclude that we are unable to reject the hypothesis that that the population means are different for 95% confidence level. The result of the t test indicates that it is not possible to statistically distinguish
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between the average delay time values obtained from AIMSUN and those from VISSIM in 95 out of 100 trials. Thus, it can be concluded that the average delay times reported by
AIMSUN and VISSIM are marginal within a 95% confidence level.
Furthermore, in general, the key differences, determined during the process of this research, between the two models can be summarized by the following factors: network coding process, simulation processing time, and data output. The network coding process of the two models are different but are both easy to use. To generate a model in
VISSIM, links and connectors were used while AIMSUN uses nodes and links to create the intersections. The same computer was used to run the simulations for the two models in order to test how fast each platform runs. AIMSUN was able to simulate the models faster than VISSIM. In addition, the output data processing in VISSIM was found to be more time consuming. AIMSUN generates Microsoft access database which can easily be transferred to a statistical program such as Microsoft Excel while VISSIM creates different file depending on the process used to generate the output and it requires more knowledge as to transferring the files into a program for analysis of the output. Overall, both platforms are easy to use and have similar capabilities. Both models are 3D capable which provide the added benefits for presentation.
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6 CONCLUSIONS
This research aimed to evaluate and compare the operational performance of four unconventional intersections: roundabout signalized roundabout, continuous flow intersection, and parallel flow intersection. The intersection designs were compared to a conventional intersection to determine which intersection can be used as an alternative when considering improvement to a conventional intersection. The specific objectives of this research were to compare the intersection under different volume conditions and quantify the effect of left turn movement on each individual unconventional intersection.
Traffic demand data was generated hypothetically to represent conditions typically encountered in the field. Three issues were considered in developing the traffic demand data. These issues were to represent both peak and off peak conditions, testing the intersection designs under balanced and unbalanced flow condition, and quantify the effect of increasing left turn volume on the performance of the intersection. A balanced flow condition considers similar volumes on all four approaches of the intersection and an unbalanced volume condition considers a major and minor road at the intersection..
The hypothetical volumes were optimized to obtain the optimum cycles and green splits for each of the flows.
To achieve the objective of this research, microscopic simulation models of each intersection was created using two separate platforms. A literature review was conducted to determine which simulation models would be capable of performing the required tasks. Aimsun 6.0 and VISSIM 5.10 professional versions were selected. 120
Microscopic models of each of the intersections were developed in both models.
The VISSIM roundabout and continuous flow intersection models were obtained from
Dr. Edara from University of Missouri and reproduced in AIMSUN to match all the characteristics for a fair comparison. The parallel flow intersection and the conventional intersection models were created in both VISSIM and AIMSUN.
After ensuring that all the models are comparable and work properly, ten replications were run using random seeds to ensure the quality and the accuracy of the output data..
Simulation time for each run was 4,800 seconds and the warm up period was set to 1,200 seconds for each model and repeated for 7,200 seconds to examine whether the run time had an effect on the results . Two measures of effectiveness: average delay time and number of stops were used to evaluate and compare the operational performance of the intersections. These measures of effectiveness were obtained from literature and past researches with the same objective of comparing the operational performance of different intersection schemes.
For the both the balanced and unbalanced scenarios, it was found that the roundabout yielded the lowest average delay time among the intersection designs. The un signalized roundabout showed average delay times less than 10 seconds per vehicle for total entering volumes of less than 2,500 vehicles per hour which, according to the level of service criteria established by the Highway Capacity Manual (2000), indicates that the level of service of the roundabout at these volumes is LOS A. At higher volumes however, the roundabout quickly reached its capacity and the average delay times were rapidly increased as the total volume increases. Introducing signals to the roundabout resulted in lower delays for flows higher than 2,500 vehicles per hour. Although the
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signalized roundabout reduced the average delays observed at the un signalized roundabout at the higher flow considered, the average delays per vehicle were still higher than the other alternatives. The parallel flow intersection showed average delays that were lower than the conventional intersections for most of the cases and similar or lower than the continuous flow intersection at low volumes. For the low and medium entering volumes, the parallel flow intersection had a LOS C while performing at a LOS E for the highest tested volume (6,000 vph). The continuous flow intersection produced the lowest delays at the medium and high entering volumes resulting in a level of service C for all the entering volumes tested.
Moreover, the effect of left turn movement was studied and quantified for each of the unconventional intersections. The results showed that increasing the left turn volume for the intersections had minimal impact on the roundabout and the parallel flow intersection.
As expected, the average delays and number of stops for the roundabout and parallel flow intersection increases as the left turning percentage is increased. Increasing the left turn percentage for the continuous flow intersection, on the other hand, reduced the average delay for the intersection at traffic volumes higher than 4,500 vehicles per hour.
A paired t test was conducted to determine whether the two microscopic platforms report significantly different average delay time values for the same intersection. A 95% confidence level ( α = 0.05) was selected for the purpose of this research. The average delay time values obtained from the two platforms for the continuous flow intersection for 15% left turn movement is selected for the statistical analysis. The result of the t test indicates that it is impossible to statistically differentiate between the average delay time values obtained from AIMSUN and those from VISSIM within 95% confidence level.
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A sensitivity analysis was performed to determine the appropriate alternative for each of the volume level considered: low (1,000 – 2,500 vph), medium (3,000 – 4,000 vph), and high (4,500 – 6,000 vph). From the sensitivity analysis, it can be concluded that the double lane roundabout would be the best alternative for locations where traffic volume is less than 3,000 vehicles per hour. Implementing signals to the roundabout can provide the benefits of reduced delay at medium volume and still perform better than the conventional intersection. For locations where the traffic volume is high and a high left turning percentage is present, the continuous flow intersection offers the best alternative.
Overall, the continuous flow intersection can be considered the best alternative compared to the other intersections evaluated in this research with a level of service C performance for all the volume scenarios tested.
6.1 Major Contributions of the Study
This research evaluates and compares the operational performance of four unconventional intersections. This research provides a guideline to traffic engineers and decision makers considering improvement to traffic operation at an underperforming conventional intersection. Depending on the problems existing at the particular location, this research can be used as a tool to select an appropriate alternative for the problem at hand.
A review of several leading microscopic simulation platforms is presented. The capabilities of each model are discussed. Moreover, two platforms: VISSIM and
AIMSUN were selected and used in this research. Since microscopic simulation has become an accepted tool in the transportation field to conduct research that aids in the
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decision making process, this paper can be used to select an appropriate microscopic simulation for different type of study.
As the parallel flow intersection gains more recognition and become accepted and implemented in the field, more research on that particular type of intersection will be required. This research can e used as reference to others considering similar type of study. A microscopic model of the parallel flow intersection was developed and can be used in future research.
6.2 Limitations of the Study
This research is a strictly theoretical and thus is subject to limitations. This research developed several theoretical models of intersections that are still being studied and for which no field data are yet available. Therefore, it was not possible to calibrate the models to represent field conditions. Additionally, traffic data were hypothetically generated and are not based on any particular location. Due to different constraints while conducting the research, only a handful of scenarios were considered. To represent all field conditions more scenarios would be needed.
6.3 Recommendations for Future Work
In order to validate the results of a simulation model calibration is a required process to ensure the quality of the output. However, field data are currently not available for some of the intersections considered. As more of these intersections are implemented in the field, it is recommended that these models be calibrated to match field conditions.
Moreover, the recommendations for future work may include the following:
Consideration of more scenarios in the absence of field data
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Calibration of models
Safety analysis to determine which intersection is the safest alternative
The use of different measures of effectiveness
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8 APPENDIX
Figure 8 1: Geometric Elements of Parallel Flow Intersection
Figure 8 2: Geometric Elements of Continuous Flow Intersection
132
Table 8 1: Average Delay Time Comparison for 20% Left Turn for the Balanced Scenarios
Total Avg. Delay Time (s/veh)
Entering Signalized
Volume Conventional Roundabout Roundabout Continuous Flow Parallel Flow
(vph) AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM
1000 18.3818 18.6218 5.6136 4.0756 9.0152 8.1729 17.2614 20.1843 15.9500 17.4813
1500 18.7806 19.8386 7.1819 6.9178 10.0707 9.8840 19.8000 20.4600 17.6372 18.0154
2000 19.3601 20.7119 10.1634 9.7034 11.7617 11.2823 20.3801 21.4671 20.1934 21.8382
2500 20.3116 21.6894 14.6442 14.0733 14.5086 14.0767 20.8238 21.4957 23.9213 23.8965
3000 22.6946 24.0118 45.3258 39.8136 20.0928 19.7642 21.5103 22.0614 25.1127 24.9400
3500 24.0462 26.7700 100.9710 84.9947 31.4071 30.4463 21.7698 22.8643 27.8137 28.0690 133
4000 28.6341 30.7974 181.8890 134.0652 46.9232 39.4621 22.2537 23.4500 28.7006 28.6172
4500 30.9061 32.0129 240.7230 203.7205 63.0627 55.8546 25.9561 23.9286 31.9473 32.1663
5000 39.3797 41.9574 336.5810 319.0191 95.1374 90.4918 27.3153 24.8171 34.0974 35.2157
6000 85.0379 86.2843 391.1070 357.8217 276.6300 255.7130 27.2180 29.1486 38.1288 39.6042
133
Table 8 2: Average Delay Time Comparison for 25% Left Turn for the Balanced Scenarios
Total Avg. Delay Time (s/veh)
Entering Signalized
Volume Conventional Roundabout Roundabout Continuous Flow Parallel Flow
(vph) AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM
1000 18.4428 18.7416 5.8612 5.2122 9.1299 8.6094 17.1483 19.4300 16.0015 18.9855
1500 19.3865 20.5421 7.6612 7.1167 10.2719 10.1771 19.6032 20.7971 18.1487 20.0051
2000 19.9657 21.0185 10.4198 10.3711 11.8933 12.0323 20.3335 21.4743 21.5463 22.5846
2500 22.3166 23.4973 16.6031 16.7733 16.9786 14.9554 20.7132 22.2557 25.3850 25.2737
3000 24.3513 26.7451 67.4825 51.7904 21.6069 20.4616 21.4187 22.8486 26.0107 26.0009 134 3500 27.8187 30.7270 134.0400 106.5730 32.6396 31.1452 21.9475 23.4357 28.3940 28.9912
4000 32.9366 33.8543 207.7130 178.4310 58.0604 50.6290 22.4017 24.8957 30.0560 30.3461
4500 42.3390 46.3757 266.7170 224.0756 76.7135 61.9033 26.6933 26.2386 33.5380 34.5164
5000 61.6746 58.6271 361.9950 342.1774 102.5970 94.1984 26.3389 29.3000 36.1784 37.8160
6000 126.9310 125.2057 418.4450 391.3644 284.0700 271.4663 26.9850 44.3914 41.5133 42.1018
134
Table 8 3: Average Number of Stops Comparison for 20% Left Turn for the Balanced Scenarios
Number of Stops (#/veh/mi)
Total Signalized
Entering Conventional Roundabout Roundabout Continuous Flow Parallel Flow
Volume (vph) AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM
1000 0.7573 0.7269 0.1337 0.0971 0.4341 0.2473 0.7360 0.9154 0.8312 0.8653
1500 0.7670 0.7387 0.2360 0.2253 0.4975 0.4887 0.8432 0.9067 0.8983 0.8942
2000 0.7863 0.7513 0.4042 0.4164 0.6148 0.6204 0.8860 0.9206 0.9450 0.9743
2500 0.7894 0.7601 0.6112 0.5835 0.8013 0.8667 0.9227 0.9217 0.9675 0.9709 135 3000 0.8000 0.7648 1.1738 0.9948 1.1632 0.9602 0.8502 0.9466 0.9885 1.0012
3500 0.8282 0.7698 2.0583 1.8367 1.3998 1.0964 0.8938 0.9511 0.9931 1.0475
4000 0.8714 0.8203 2.8426 2.5567 1.6880 1.3235 0.9334 0.9570 1.0092 1.1850
4500 0.9141 0.8591 3.3998 3.0948 2.0394 1.8106 1.1039 0.9610 1.2611 1.4412
5000 0.9544 0.8921 3.4676 3.2110 2.6807 2.2411 1.1628 0.9863 1.4482 1.6116
6000 1.4469 1.3453 4.3209 3.8864 3.8208 3.3413 1.1386 1.0708 1.5355 1.7564
135
Table 8 4: Average Number of Stops Comparison for 25% Left Turn for the Balanced Scenarios
Number of Stops (#/veh/mi)
Total Signalized
Entering Conventional Roundabout Roundabout Continuous Flow Parallel Flow
Volume (vph) AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM AIMSUN VISSIM
1000 0.7603 0.7318 0.1560 0.1043 0.4440 0.2711 0.7387 0.9234 0.8868 0.8974
1500 0.7817 0.7416 0.2649 0.2527 0.5090 0.4890 0.8506 0.9249 0.9242 0.9442
2000 0.8007 0.7634 0.4133 0.4142 0.6184 0.5765 0.8887 0.9226 0.9683 0.9943
2500 0.8150 0.7700 0.6888 0.6952 0.9035 0.8864 0.9309 0.9430 0.9817 0.1002 136 3000 0.8342 0.7845 1.5061 1.4364 1.3247 0.9806 0.8608 0.9763 0.9905 1.0761
3500 0.9032 0.8145 2.3217 2.1134 1.4677 1.1994 0.9030 0.9854 1.0004 1.1675
4000 0.9466 0.8843 3.0614 2.0876 1.7825 1.4684 0.9457 0.9911 1.2542 1.4885
4500 1.0963 0.9221 3.7635 3.2250 2.0905 1.9567 1.1446 1.0246 1.4148 1.6376
5000 1.2612 1.1028 3.6288 3.4417 2.8670 2.8110 1.1332 1.0697 1.5130 1.7416
6000 2.1978 1.5290 4.9593 4.1192 4.1818 3.7635 1.1499 1.4130 1.7007 1.8660
136
Conventional AIMSUN Avg DelayTime Comparison for 25% Left Turn Roundabout
450.0000 Signalized Roundabout 400.0000 CFI 350.0000 PFI 300.0000 250.0000 200.0000 150.0000 100.0000 Avg. Delay Time Avg. Delay (s/veh) 50.0000 0.0000 1000 2000 3000 4000 5000 6000 Total Enetring Volume (vph)
Figure 8 3: Average Delay Time Comparison for 25% Left Turn Movement for Results Obtained
from AIMSUN 6.0
Conventional AIMSUN Avg Number of Stops Comparison for 25% Left Turn Movement Roundabout 5.0000 Signalized 4.5000 Roundabout CFI 4.0000
3.5000 PFI
3.0000
2.5000
2.0000
1.5000
1.0000
Average Number of Stops (#/veh/mi) Stops of Number Average 0.5000
0.0000 0 1000 2000 3000 4000 5000 6000 Total Entering Flow (vph)
Figure 8 4: Average Number of Stops Comparison for 25% Left Turn Movement for Results
Obtained from AIMSUN 6.0
137
Conventional VISSIM Avg DelayTime Comparison for 25% Left Turn Roundabout
400.0000 Signalized Roundabout 350.0000 CFI
300.0000 PFI 250.0000
200.0000
150.0000
100.0000 Avg. Delay Time Avg. Delay (s/veh) 50.0000
0.0000 1000 2000 3000 4000 5000 6000 Total Enetring Volume (vph)
Figure 8 5: Average Delay Time Comparison for 25% Left Turn Movement for Results Obtained
from VISSIM 5.10
Conventional VISSIM Average Number of Stops Comparision for 25% Left Turn Volume Roundabout
4.5000 Signalized Roundabout 4.0000 CFI
3.5000 PFI
3.0000
2.5000
2.0000
1.5000
Number of Stops (#/veh/mi) Stops of Number 1.0000
0.5000
0.0000 1000 2000 3000 4000 5000 6000 Total Entering Volume (vph)
Figure 8 6: Average Number of Stops Comparison for 25% Left Turn Movement for Results
Obtained from VISSIM 5.10
138
Conventional Intersection Avg. Delay Time
140
120
100
80 15% Left Turn (s/veh) 20% Left Turn (s/veh) 60 25% Left Turn (s/veh)
40 Avg. Delay Time Avg. Delay (s/veh)
20
0 0 1000 2000 3000 4000 5000 6000 7000 Total Entering Flow (vph)
Figure 8 7: Average Delay Time Comparison for Parallel Flow Intersection for Different Left Turn
Movement
Conventional Intersection Avg Number of Stops
2.5
2
1.5 15% Left Turn (#/veh/mi) 20% Left Turn (#/veh/mi) 25% Left Turn (#/veh/mi) 1
0.5 Avg. Number Avg. of Stops (#/veh/mi)
0 0 1000 2000 3000 4000 5000 6000 7000 Total Entering Flow (vph)
Figure 8 8: Average Number of Stops Comparison for Conventional Intersection for Different Left
Turn Movement
139
Roundabout Avg. Number of Stops
6
5
4
15% Left Turn (#/veh/mi) 3 20% Left Turn (#/veh/mi) 25% Left Turn (#/veh/mi)
2
1 Avg. Avg. of Number Stops (#/veh/mi)
0 0 1000 2000 3000 4000 5000 6000 7000 Total Entering Flow (vph)
Figure 8 9: Average Number of Stops Comparison for Roundabout for Different Left Turn
Movement
CFI Avg. Number of Stops
1.4
1.2
1
0.8 15% Left Turn (#/veh/mi) 20% Left Turn (#/veh/mi) 0.6 25% Left Turn (#/veh/mi)
0.4
Avg. Avg. Number of Stops (#/veh/mi) 0.2
0 0 1000 2000 3000 4000 5000 6000 7000 Total Entering Flow (vph)
Figure 8 10: Average Number of Stops Comparison for Continuous Flow Intersection for Different
Left Turn Movement
140
PFI Avg. Number of Stops
1.8000
1.6000
1.4000
1.2000
1.0000 15% Left Turn (#/veh/mi) 20% Left Turn (#/veh/mi) 0.8000 25% Left Turn (#/veh/mi)
0.6000
0.4000
Avg. Avg. Number Stops of (#/veh/mi) 0.2000
0.0000 0 1000 2000 3000 4000 5000 6000 7000 Total Entering Flow (vph)
Figure 8 11: Average Number of Stops Comparison for Parallel Flow Intersection for Different Left
Turn Movement
141