Technical Memorandum #2
Ramp Management Strategy Technical Memorandum
DRAFT
Interstate 235 Ramp Management Feasibility Study
April, 2014
HRG Project Number: 40120022.06
Prepared For:
Prepared By:
HR Green, Inc. Interstate 235 Ramp Management Ramp Management Strategy Technical Memorandum - DRAFT Feasibility Study
PREFACE
The Interstate 235 Ramp Management Feasibility Study has been completed in a system of technical memorandums documenting the project. These technical memorandums are compiled to form the final project deliverable.
Technical Memorandum #1: Existing Conditions Review
The first of the three memorandums produced documents the existing conditions of the I-235 study corridor. The existing conditions review included a high-level review of crash history, entrance ramp geometrics, documentation of traffic volume data, documentation of travel time data, and completion of traffic operations analysis.
Technical Memorandum #2: Ramp Management Strategy Review
The second memorandum reviews three components. The first component is the overall ramp management strategy which includes evaluation of isolated ramps versus a group strategy and assessment of metering timing parameters. The second component is the evaluation of I-235 corridor operations for existing and future conditions, with and without the use of ramp metering. The final component is the ramp meter design criteria including whether there is a single lane or dual lane and if dual, whether a dual/simultaneous vehicle release or alternating vehicle release strategy is utilized. In addition, stop bar placement, pavement marking/signing modifications and ITS infrastructure improvements are discussed. The strategy is developed based on the findings of the first memorandum titled Existing Conditions Review Technical Memorandum.
Technical Memorandum #3: Benefit/Cost Analysis
Based on the ramp management strategies and locations identified within the second technical memorandum, a review of the anticipated user benefit and implementation/operating costs of the selected ramp metering concept is completed. The goal of the Benefit/Cost Technical Memorandum is to quantify the relative benefit of implementing the ramp management strategy along the I-235 corridor.
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TABLE OF CONTENTS I. INTRODUCTION ...... TM 2-1 II. RAMP MANAGEMENT STRATEGY ...... TM 2-2 A. Conditions Addressed by Ramp Metering ...... TM 2-2 B. Criteria for Ramp Metering ...... TM 2-3 C. Geographic Extent and Ramp Metering Approach ...... TM 2-4 D. Ramp Metering Algorithms ...... TM 2-7 E. Queue Adjustments and Flow Rates ...... TM 2-10 III. RAMP METER OPERATIONS ANALYSIS ...... TM 2-13 A. Existing Conditions Analysis Summary ...... TM 2-13 B. Ramp Meter Operations Analysis ...... TM 2-15 1. Methodology ...... TM 2-15 2. Traffic Scenarios ...... TM 2-15 3. Ramp Meter Timings ...... TM 2-16 4. Evaluation of Operations ...... TM 2-19 5. Summary of Operations Analysis ...... TM 2-26 IV. RAMP METER DESIGN CRITERIA ...... TM 2-27 A. Sketch Planning Spreadsheet Tool ...... TM 2-27 B. Acceleration/Storage Length ...... TM 2-27 C. Stop Bar Placement ...... TM 2-32 D. Single vs. Dual Lane Operation ...... TM 2-33 E. Dual Lane Vehicle Release Operations ...... TM 2-36 F. Pavement Marking/Signing Modifications ...... TM 2-37 G. ITS Infrastructure Improvements ...... TM 2-38 V. SUMMARY OF FINDINGS ...... TM 2-40 VI. APPENDIX 2A: OPERATIONAL ANALYSIS ...... TM 2-A VII. APPENDIX 2B: MnDOT DESIGN CRITERIA INFORMATION ...... TM 2-B
LIST OF TABLES Table II-1: Entrance Ramp 2013 Peak Hour Volumes ...... TM 2 - 12 Table III-1: Ramp Meter Boundary Analysis I-235 Westbound (20 Year Traffic) ...... TM 2 - 17 Table III-2: Ramp Meter Timings I-235 Westbound (20 Year Forecasts) ...... TM 2 - 18 Table III-3: Ramp Meter Timings I-235 Westbound (20 Year Forecasts) ...... TM 2 - 19 Table III-4: East-West Segment - AM Peak Period Travel Time Improvements ...... TM 2 - 22 Table III-5: East-West Segment - PM Peak Period Travel Time Improvements ...... TM 2 - 23 Table IV-1: List of Entrance Ramps with Inadequate Acceleration Length ...... TM 2 - 28
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LIST OF EXHIBITS Exhibit II-1: Time Trends for Speed and Flow (Typical Morning Rush) ...... TM 2-3 Exhibit II-2: Summary of Ramp Metering Approaches ...... TM 2-5 Exhibit II-3: Summary of Ramp Metering Approach Advantages and Disadvantages ...... TM 2-6 Exhibit II-4: Ramp Metering Flow Controls ...... TM 2-11 Exhibit III-1: Short-Term Deficiencies on the I-235 Corridor ...... TM 2-14 Exhibit III-2: Average Weekday Peak Period Volume Comparisons ...... TM 2-16 Exhibit III-3: East-West Segment - AM & PM Peak Period Travel Times (20 Year Forecast) ...... TM 2-20 Exhibit III-4: East-West Segment - AM Peak Travel Times with and without Metering .... TM 2-24 Exhibit III-5: East-West Segment - PM Peak Travel Times with and without Metering .... TM 2-25 Exhibit IV-1: Ramps with Inadequate Acceleration Length ...... TM 2-29 Exhibit IV-2: Typical Metered Entrance Ramp Layout ...... TM 2-33 Exhibit IV-3: Example DOT Typical Cross Sections ...... TM 2-35 Exhibit IV-4: Typical Vehicle Release Strategies ...... TM 2-36 Exhibit IV-5: Typical Entrance Ramp Metering Communications System Layout ...... TM 2-39
CERTIFICATIONS
Under Compendium Overview Section
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I. INTRODUCTION
The purpose of the series of technical memorandums is to study the feasibility of implementing ramp metering along the Interstate 235 (I-235) corridor located in Des Moines, Iowa. This technical memorandum, the second of the three memorandums, will:
Provide and an overview of the overall ramp management strategy including evaluation of isolated ramps versus a group strategy and assessment of metering timing parameters.
Review the existing and future I-235 corridor operations with and without the use of ramp metering.
Provide an overview of the proposed ramp meter design criteria including a single lane or dual lane operation, and if dual, whether a dual/simultaneous vehicle release versus alternating vehicle release strategy is utilized. In addition, stop bar placement, pavement marking/signing modifications and ITS infrastructure improvements are discussed.
Ramp metering controls ramp traffic with traffic signal device(s) on the ramp in order to reduce freeway mainline congestion and crashes. It is generally considered the most flexible method of freeway mainline improvement among the four management strategies which include; ramp closure, ramp metering, special use treatment, and ramp terminal treatment. The benefits of ramp metering include increased/improved system operation on the mainline (increase vehicle throughput and travel speed), reduced number of crashes and improved environmental impact. Improved throughput and faster speeds on the mainline may result in some additional delay on the ramps themselves. When implemented properly, this delay will be more than offset by the mainline improvements resulting in faster trip times for most users.
Different ramp metering strategies such as local/system-wide metering and pre- time/traffic responsive metering can be implemented based on the study goals and objectives, as well as characteristics of the study freeway-ramp system.
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II. RAMP MANAGEMENT STRATEGY
The ramp metering implementation strategies selected are a function of operational goals and objectives, traffic patterns, location and nature of bottlenecks and physical layout of the roadways and ramps.
As noted elsewhere in the documents developed for this study, ramp metering strategy, in order to be effective, must address the goals and objectives of the implementing agency and the specific corridor(s) under study. The goal of the I-235 corridor, like many metering operations, is to achieve a balance between freeway mainline improvements, including both travel time improvements and crash reduction, and queuing delay times on entrance ramps. Ramp meter operational strategies are designed to strike the optimal balance, given the physical constraints of the system.
There are a number of key operational decisions that are required in the implementation of ramp metering. One of the first decisions reflects whether the major objective is to reduce severe congestion or safety problems at specific locations or to provide a series of improvements across an entire corridor. The former would indicate a decision for isolated or scattered ramp metering, while the latter would point toward a more systematic approach. There are other factors that need to be considered in identifying an overall strategy including:
Characteristics of surface street network and potential diversion impacts Spacing of ramp/surface intersections Ramp lengths Proximity of on- and off-ramps and merge/weave conditions Distribution, nature and severity of crashes Capability to implement a central control system Operational resources available to adjust timings
The FHWA Ramp Metering and Control Handbook, FHWA, January 2006, provides the most comprehensive summary of operational strategies. This section includes a number of figures and graphs from the handbook that help to illustrate operational conditions and strategies.
A. Conditions Addressed by Ramp Metering
In order to successfully identify and implement operational strategies it is important to understand and communicate the conditions that occur when mainline flows exceed the capacity of the roadway. As mainline flows increase, density increases with a corresponding decrease in traffic speed. As traffic demand approaches freeway capacity, traffic flow begins to deteriorate. This increases the probability of flow breakdown (i.e., transition from a stable state to a congested state). This concept is illustrated in Exhibit II-1.
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Exhibit II-1: Time Trends for Speed and Flow (Typical Morning Rush)
(Ramp Metering and Control Handbook, FHWA, January 2006, Chapter 5, Figure 5-1)
An important note is that unmetered platoons entering a freeway can cause disruptions in situations where traffic is heavy but still free flow. Platoons interfering with traffic trying to weave across the roadway and exit can also cause safety problems. These are conditions that do exist on portions of I-235. The handbook notes that metering can help alleviate these conditions in two ways:
Reduce the flow at metered ramps during certain time periods and redistribute it to later time periods. This reduces the flow at critical times to reduce congestion at merge points and at downstream bottlenecks. Change driver behavior. These changes include the time of day that metered ramps are accessed, the ramp they access, or their overall selected route. Some may also change mode of travel, but this is a relatively small proportion of the overall ramp traffic.
B. Criteria for Ramp Metering
In selecting a strategy one must consider each of the following elements:
1. Geographic extent – the area that will be covered by ramp metering and whether the meters in that area will be operated in an isolated manner or as part of a larger system of meters. 2. Ramp metering approach – local or system-wide and pre-timed or traffic responsive. 3. Metering algorithm – the specific logic and calculations used to select or determine a metering rate. 4. Queue management – how the metering rate will be affected by ramp queues and how the agency will keep queues at a manageable and acceptable level.
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5. Flow control – how traffic will be released from the meter, one at a time or two at a time in one lane or multiple lanes. 6. Signing – how drivers will know that a ramp meter is on or off.
Items 1 and 2 are the main focus of this study and should be settled in the planning phase of the project. Items 4 and 6 are added as key issues during the design phase. Items 3 and 5 are primarily operational, although the decisions made in item 5, regarding flow control, will have implications for the design as well.
C. Geographic Extent and Ramp Metering Approach
Ramp metering can be deployed at individual ramps to address specific safety or congestion problems. This strategy can be applied on freeways which have limited congestion but experience a small number of bottlenecks that might result from closely spaced on and off-ramps or small stretches of freeway that are over capacity. The advantage of this strategy is that it can address a specific problem at relatively low cost. A disadvantage is that motorists may divert to other ramps to avoid a single meter, adding traffic to surface streets and reducing the effectiveness of the improvement.
Isolated strategy can also be used as demonstration of concept or where a congestion problem is limited to a single or small number of locations – This strategy is similar to the isolated ramp strategy described above but has a different objective; to demonstrate the feasibility of ramp metering in anticipation of further deployment. This strategy provides a low-cost method of testing the impacts of ramp metering, including motorist response.
As noted motorists may have a tendency to avoid an isolated metered ramp. Diversion could result in increased congestion on surface streets or other ramps, thus negating the benefits of the meter. An isolated strategy also makes it difficult to educate the public regarding ramp metering, as the target audience is relatively small and difficult to reach. A general educational campaign would waste resources since many of those receiving the information would not ever use the metered ramp. The isolated strategy will encourage diversion if there are feasible paths to other interchanges.
While isolated strategies are relatively low-cost, the group strategy takes advantage of economies of scale, thus reducing cost per interchange. Active management of an isolated meter can be expensive; while the group strategy makes it economical to assign personnel either full- or part-time to actively manage the system and adjust metering rates as needed.
In general the group strategy is more effective when operating conditions are similar across a corridor. Ramp metering can help to maintain a consistent flow and thus avoid extensive diversion to surface streets. Crash reduction resulting from metering strategies also helps in this regard. Where operating conditions vary significantly it is helpful to have a system that can adjust in real time, in order to optimize under unstable conditions.
Exhibit II-2 addresses both items 1 and 2, the geographic extent of the metering system and the ramp metering approach. The matrix identifies key criteria that may impact this decision.
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Exhibit II-2: Summary of Ramp Metering Approaches
(Ramp Management and Control Handbook (FHWA, January 2006))
In addition to the grouping strategy, the metering rate design parameter defines the number of vehicles allowed to enter freeway in one hour. Pre-timed or fixed time metering uses a fixed entering rate based on historical data, and is typically used to address recurring congestion and local safety problems. A pre-timed system does not require the detection of mainline traffic flow, and therefore is not able to adapt to varying traffic flow conditions, which can lead to unnecessary queuing and the requirement of continual monitoring by DOT personnel.
Exhibit II-3 summarizes the advantages and disadvantages of four approach scenarios; pre-timed metering both local and system-wide (same set of advantages and disadvantages for both), system-wide metering and local traffic responsive metering. Local traffic responsive metering focuses on areas near each ramp while system-wide metering attempts to optimize flow over a longer stretch of freeway.
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Exhibit II-3: Summary of Ramp Metering Approach Advantages and Disadvantages
(Ramp Management and Control Handbook (FHWA, January 2006))
The analysis conducted for the I-235 study focuses on system-wide, pre-timed metering. While existing and projected deficiencies are more pronounced in certain portions of the corridor, they are widespread enough to consider system- wide metering. The operational analysis assumed pre-timed metering based on ramp and mainline demand This was helpful in identifying the impact of varying ramp demand across the corridor, impacts on ramp design and capacity and impact on upstream intersections. Further analysis could easily consider the
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impacts of traffic-responsive metering; this analysis would be more effective with the use of a microsimulation model.
D. Ramp Metering Algorithms
Ramp metering algorithms are used to help implement responsive metering strategies. Potential algorithms may be specified in the design phase to make sure that equipment and software can address the desired strategies. It can be expected however that they will be tested during an early operational phase and may be modified depending on the outcome. Detailed microsimulation models can be used during the design phase to test the performance of various algorithms. Recent ramp meter implementations have been accompanied by educational and outreach programs that encourage motorists to stay with their existing travel patterns. However, it is impossible to know for certain what the public response will be until the program is in place. Some of the common metering algorithms are described briefly below. More detailed information can be found in the Ramp Management and Control Handbook: (http://ops.fhwa.dot.gov/publications/ramp_mgmt_handbook/manual/manual/5_1.htm#534)
Minnesota Zone Algorithm
The Minnesota Zone Algorithm, a stratified zone metering algorithm, attempts to balance traffic volumes entering and exiting predetermined metering zones to maintain a consistent flow of traffic from one zone to another. The algorithm incorporates entering and exiting traffic volumes of each zone and adjusts the metering rate at individual ramps to hold traffic as needed to maintain consistent traffic flow on the mainline. The algorithm selects one of six predetermined metering rates, ranging from no metering to a cycle length of 24 seconds (meter rate of 150 veh/h).
Metering zones are typically three to six miles in length, and may include several ramps that are not metered. The upstream portion of each zone is typically a free flow area not subject to high incident rates. The downstream portion of a zone typically includes areas defined as bottlenecks, where demand is the greatest.
Key features of the Minnesota Algorithm are:
Ramp queue lengths are calculated based on queue detector measurements. The queue waiting time is limited to a prescribed value (e.g. four minutes), and the ramp meter rate is raised, as necessary to assure that this condition is met. Filtered mainline loop detector data at 30-second intervals is used for the meter rate setting algorithm. Spare capacity is calculated from mainline measured volume and speed data. Meters are grouped into zones. The intent of the metering algorithm is to restrict the total number of vehicles entering a zone to the total number leaving (including spare capacity). Zones are organized by “layers”. Higher-level layers feature larger zones with greater overlap among zones.
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Metering rates are calculated by distributing the spare capacity among the meters in a zone. If the required metering rates are lower than the minimum metering rates allowed, the metering rates are recalculated for the next higher layer. This process is repeated until all of the minimum rates are satisfied.
Seattle Bottleneck Algorithm
The Seattle Bottleneck Algorithm calculates both a local control metering rate and a bottleneck metering rate. Calculation of the bottleneck rate occurs when both the following conditions are met:
Threshold occupancy is exceeded. Vehicles continue to be stored in the section.
When conditions are not met, just the local control metering rate is determined.
The local metering rate is based on mainline occupancy adjacent to the metered ramp. For every metered ramp, a meter rate/mainline occupancy relationship is defined by five occupancy-metering rate pairs. The algorithm compares mainline occupancy adjacent to the ramp to pre-defined occupancy-metering rate pairs. The metering rate is determined by interpolating between these pairs for the actual mainline occupancy.
The bottleneck rate is based on traffic volumes downstream of the ramp. A specific number of upstream ramps are identified for every freeway segment, defined by two adjacent mainline detector stations. The bottleneck metering rate reduces the number of vehicles entering the mainline from these ramps by the number of vehicles stored in the freeway segment. Each ramp may have multiple bottleneck metering rates calculated, one for each downstream segment for which it has been identified. The algorithm selects the most restrictive of these as the final bottleneck metering rate.
The algorithm compares the final bottleneck metering rate to the local metering rate and selects the more restrictive of the two. The final step is to adjust the metering rate for ramp conditions, such as queuing. Two queue detection loops are located on each ramp. If traffic queues onto either of these, the metering rate is adjusted upward so the queuing can be eased. A larger adjustment is applied when queues reach the queue detector farthest back from the ramp meter. The final adjusted metering rate is implemented for each ramp.
Fuzzy Logic Algorithm
This algorithm was developed to address deficiencies in the Seattle Bottleneck algorithm. Fuzzy logic has the ability to address multiple objectives (by weighing the rules that implement these objectives) and to implement the tuning process in a more user-friendly fashion. Rule groups used by the algorithm include:
Local mainline speed and occupancy. Downstream speed and occupancy.
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Ramp queue occupancy. Quality of the ramp merge.
Inputs include speed and occupancy from the mainline and downstream detector stations, the queue occupancy detector and the advanced queue occupancy detector (at the upstream end of the ramp storage location). Each numerical IS input into “fuzzy classes”. For local occupancy and local speed, the fuzzy classes used are very small (VS), small (S), medium (M), big (B), and very big (VB). The degree of activation indicates how true that class is on a scale of 0 to 1. For example, if the local occupancy were 20 percent, the medium class would be true to a degree of 0.3, and the big class would be true to a degree of 0.8, while the remaining classes would be zero. Other criteria include downstream occupancy, downstream speed, queue occupancy and advance queue occupancy. For each input at each location, the dynamic range, distribution and shape of these fuzzy classes can be tuned.
Denver, Colorado Helper Algorithm
The Denver, Colorado Helper Algorithm is based on a local traffic responsive algorithm with centralized control. Under centralized control, meters are polled every 20 seconds to collect detector and metering data. If the meter is operating at its most restrictive metering rate and if the detector’s threshold occupancy value is exceeded, the algorithm defines the meter as “critical”. Based on this classification, the algorithm begins to override upstream ramp control. If a ramp remains critical for more than one minute (three consecutive, 20-second periods), the algorithm reduces at the next upstream meter by one metering rate level. The algorithm continues this process for every meter within the system for each consecutive 20-second period until the problem is resolved or until all ramps have been overridden.
The algorithm assigns up to seven ramp meters to as many as six groups or zones (maximum of 42 ramp meters).
Northern Virginia Algorithm
The Northern Virginia Algorithm bases the meter rate in a particular “zone” on predicted demands. The algorithm defines a link as the freeway segment between two entrance ramps. Metering zones can include up to ten links.
The meter rate is determined as the difference between the predicted demand and the capacity of the link that contains the ramp. The predicted arrival demand is calculated sequentially in each zone starting at the link furthest upstream in the zone. The available capacity is sequentially calculated in each zone starting at the link furthest downstream in the zone.
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SWARM Algorithm
The System-Wide Area Ramp Metering (SWARM) Algorithm is used for coordinated, system-wide metering approaches. The SWARM Algorithm essentially is the product of two independent control algorithms collectively referred to as SWARM1 and SWARM2.
SWARM1, the more complex of the two, uses previously recorded data to forecast future volumes. Based on this forecast, SWARM1 determines the onset of congestion and restricts real-time volumes from exceeding pre-determined saturation values. SWARM2 is basically a local traffic responsive algorithm. The overall SWARM algorithm compares the metering rates of both SWARM1 and SWARM2 and picks the more restrictive of the two.
E. Queue Adjustments and Flow Rates
Two other key considerations in ramp timing are queue adjustments and flow control. Queue adjustments are generally incorporated into ramp metering algorithms or an adjustment process is added to make sure that queues do not back up into surface intersections. If sufficient physical storage cannot be provided for worst-case queues, ramp queues must be detected and metering rates adjusted to prevent backups. The adjustment process generally follows these steps:
Queue detectors are placed on ramps upstream of the meter stop bar at critical locations. If a queue is detected at the detector, the meter rate is increased. Some algorithms will increase at one level when the queue first extends to the detector and increase the metering rate at a higher level if the queue still exists after a programmable amount of time. Some systems have a second queue detector further upstream that will cause the metering rate to increase sharply to more quickly reduce the queue length. Some algorithms take the increased metering rate caused by ramp queues at one ramp into account at other ramps and will adjust those metering rates downward to try to keep the level of traffic on the freeway close to the pre-queue adjustment level. Some algorithms, like the fuzzy logic algorithm, use queues as an integral part of the algorithm that calculates the metering rate.
Flow control refers to the manner and rate by which vehicles are allowed to enter a freeway from a ramp meter. The theoretical maximum rate that vehicles merge with traffic on a freeway facility and the length of queues that result from metering applications is in part a result of the type of flow control implemented at the ramp. The selection of a flow rate depends on several factors. These factors include ramp length, number of lanes, and traffic volume.
There are three strategies for controlling the flow of vehicles entering freeway facilities from a ramp. One vehicle per green allows a single vehicle to go through on each green and can accommodate a maximum flow of 900 vph. Cycle lengths are generally 4-5 seconds. Longer green cycles that allow around 6 seconds or more can be
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implemented to allow 2 or more vehicles to go through the green cycle. Capacity per hour is approximately 1100-1200 vph. Finally, tandem flow allows two lanes of traffic to enter, generally in staggered fashion to facilitate merging. This strategy can accommodate up to 1700 vph.
Different flow control strategies along with associated typical traffic capacities are described in Exhibit II-4.
Exhibit II-4: Ramp Metering Flow Controls
(Ramp Management and Control Handbook (FHWA, January 2006))
Table II-1 provides a list of the thirty-six entrance ramps along with the existing AM and PM peak hour traffic volumes.
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Table II-1: Entrance Ramp 2013 Peak Hour Volumes Eastbound/Northbound AM PM Westbound/Southbou AM PM Peak Peak nd Peak Peak Hour Hour Hour Hour 1 50th Street 950 1310 20 WB-SB E. Euclid 580 260 Avenue 2 SB-EB Valley West Drive 240 890 21 EB-SB E. Euclid 170 270 Avenue 3 NB-EB Valley West Drive 360 200 22 Guthrie Avenue 400 280 4 22nd Street 620 840 23 E. University Avenue 1560 680 5 73rd Street 390 730 24 E. 14th Street 890 710 6 63rd Street/Iowa Hwy 28 890 710 25 E. 6th Street 360 960 7 56th Street 530 250 26 2nd Avenue 460 720 8 SB-EB 42nd Street 260 170 27 7th Street 410 1120 9 NB-EB 42nd Street 180 100 28 Keosauqua Way 380 1020 10 31st Street 450 390 29 19th Street 590 1530 11 Keosauqua Way 540 1420 30 Martin Luther King 210 240 Parkway 12 5th Avenue 210 1260 31 31st Street 530 560 13 2nd Avenue 280 800 32 42nd Street 670 560 14 Pennsylvania Avenue 150 630 33 63rd Street 740 480 15 E. 15th Street 620 820 34 73rd Street 580 630 16 Easton Boulevard 180 180 35 22nd Street 360 470 17 Guthrie Avenue 240 330 36 Valley West Drive 290 590 18 EB-NB E. Euclid Avenue 140 260 19 WB-NB E. Euclid Avenue 250 280
From review of the entrance ramp peak hour volumes in conjunction with Exhibit II-4, the entrance ramps can generally be served using single lane / one vehicle per green flow control strategies. A small number of westbound ramps experience volumes over 900 vph during the PM peak hour and thus may require more aggressive strategies than single lane/one vehicle per green. As these are located close to each other, diversion to other ramps is not feasible. Options to expand ramp capacity to two lanes, should be considered in the design phase, particularly at the 19th Street ramp. If dual-lane operation cannot be physically accommodated than traffic-responsive metering should be implemented to allow multiple vehicles per green when necessary. At times when ramp queues threaten to back up in the surface intersection, ramp flow can be increased, recognizing that some speed reduction on the mainline may occur.
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III. RAMP METER OPERATIONS ANALYSIS
A. Existing Conditions Analysis Summary
The ramp analysis results for existing conditions indicate that there were both isolated and clustered failing LOS freeway segments for both directions during both AM and PM periods. As defined in the Existing Conditions Review Technical Memorandum, three categories were used to assist the understanding of the freeway operation results. Based on the LOS results, roadway segments along the I-235 corridor were placed in one of three classifications:
Current deficiency – Segments that have current LOS D or E or lower during part of the day, as well as segments with higher than average crash rates and clear geometric deficiencies. These segments would be candidates for ramp metering in the 1-5 year range.
Potential short-term deficiency – Segments that are currently operating satisfactorily but are close to LOS D or crash rate thresholds that could justify ramp metering. These segments would become deficient with relatively small amounts of growth and thus should be considered for deployment in the 5-10 year range.
Satisfactory operation – Segments that are operating satisfactorily and are likely to continue to do so in the future. It should be noted that some of these segments may be metered in order to achieve continuity of the system; for example a segment that is satisfactory but located in between two deficient segments.
The status of segments along the study corridor is documented in Exhibit III-1 below. In the eastbound direction, there are no current deficiencies but potential short-term deficiencies exist in two areas, from 22nd Street to 63rd Street (2 entrance ramps) and from Pennsylvania Avenue to East Euclid Avenue (3 entrance ramps).
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Exhibit III-1: Short-Term Deficiencies on the I-235 Corridor
In the westbound direction, current deficiencies are found between 42nd Street and Valley View Drive (4 entrance ramps) and on a shorter stretch between 6th Street and Keosauqua Way (2 entrance ramps). Since single metering cannot result in acceptable traffic conditions, system-wide metering should be used. There are also potential westbound short-term deficiencies east of 42nd Street as well between University Avenue and Pennsylvania Avenue.
Based on the existing conditions analysis two key decisions were made regarding the operational analysis:
The corridor was divided into two major segments, the east-west portion and the north-south portion, with the split occurring at University Avenue. This decision was made primarily based on the differences in westbound flow and the likelihood that the western portion of the corridor may experience a higher rate of growth.
Conditions in the corridor appeared to lend themselves more to a system- wide approach rather than an isolated approach. The corridor does not have segments with extreme congestion or safety problems but faces potential increases in congestion over a period of time.
Details of the analysis are discussed within the following Ramp Meter Operations Analysis section of this technical memorandum.
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B. Ramp Meter Operations Analysis
1. Methodology The operations analysis was conducted using the FREEVAL (FREeway EVALuation) tool which was first developed as a computational engine for the Highway Capacity (HCM) freeway facilities methodology chapter in 2000. It has since gone through several improvements and the latest FREEVAL 2010 is now executed in a Microsoft Excel and Visual Basic programming platform. It is designed for the analysis of freeway facilities including ramp metering. The analysis conducted for this study utilized existing conditions data, including volume and speed/travel time data collected in 15-minute increments. The analysis included: The analysis included three time periods: existing conditions, a 5 year horizon (2019) and a 20 year horizon (2034). Analysis included three-hour AM and PM peak periods. The analysis included scenarios with and without ramp metering in order to compare the impacts. Ramp meter timings were set based on demand for each entrance ramp. Timings were set to make sure that all ramp traffic was serviced without queue backups into the surface street network. Measures of effectiveness used to evaluate operations include vehicle-hours of travel, travel time, speeds and level of service. Vehicle hours of travel and speeds were also input into the TOPS-BC benefit/cost analysis (documented in the third technical memorandum titled, Benefit/Cost Technical Memorandum). Crash data were used in TOPS-BC as well. Evaluation measures are shown in a series of charts and graphs and are documented in the following sections.
2. Traffic Scenarios Traffic scenarios were developed by applying growth rates to existing traffic counts collected in July 2013. The growth rates were derived by examining both the regional demand modeled growth and historical growth trends. The estimated regional model growth rates in the corridor are modest; ranging from 1.4% to 1.6% annually. Recently the east-west segment has grown faster than projections, significantly exceeding the 1.4% per year. Therefore, a 2% rate, compounded annually, was used for the east-west segment. Historical weekday traffic was examined at the permanent count station on I-235 south of Guthrie Avenue. The purpose was to determine how the July counts that were collected compared to other months of the year. Exhibit III-2 below illustrates the 2012 average weekday peak period traffic variation between the low, high average volumes and the average traffic for the month of July. As indicated in the chart, the July volumes fell in between high and low average conditions, as derived from the permanent count station. Therefore, no adjustment was required for seasonal variation.
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Exhibit III-2: Average Weekday Peak Period Volume Comparisons
3. Ramp Meter Timings Ramp meter timings for this feasibility study were developed based on 15-minute demand volumes on the freeway mainline and the entrance ramps. The ramp metering approach based on the analysis tool being used will be pre-timed signals with rates adapting every 15 minutes based on straightforward assumptions. As a ramp metering project is carried forward into implementation, more involved analysis work should be conducted including testing of timings and algorithms within a microsimulation model. The steps for developing ramp timings included the following: 1. Identify entrance ramps and time periods to be metered 2. Develop ramp meter timings 3. Review queue lengths The following discussion illustrates the process used to develop timings using westbound I-235 20-year projection traffic volumes as the case study. The complete set of analysis can be found in Appendix 2A. Identify Entrance Ramps and Time Periods to be Metered: The first step taken was to identify a time and spatial window in which ramp meters should be deployed. The 15 minute demand immediately downstream of the entrance ramps were compared to a simple capacity assumption. If the demand exceeded the capacity for that time period, the location was identified as requiring a ramp meter. Table III-1 includes mainline volumes by segment and illustrates the identification of where (which entrance ramps) and when (what time periods) should have ramp metering deployed. The top line shows mainline planning capacity, which varies by ramp based on entrance ramp length and mainline volume. The highlighted cells show the locations and time periods where mainline volumes are high enough to justify ramp
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metering operation. Metering would be considered west of 6th Street between 3:30 PM and 6:00 PM with peak demand occurring between 19th Street and 73rd Street around 4:30 to 5:15 PM. Similar analyses were conducted for both peak periods for the existing, 5-year and 20-year periods.
Table III-1: Ramp Meter Boundary Analysis I-235 Westbound (20 Year Traffic)
Note: The Red box identifies the entrance ramps and time periods requiring ramp metering.
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Develop Ramp Meter Timings: Ramp meter timings were developed for the feasibility study based on demand but also on the premise that the entrance ramp volumes would not be overly managed with extreme rates. While an important goal is to make sure that entrance ramp storage is not exceeded, this must be balanced against the need to keep traffic on the freeway moving and not encourage excessive diversion to surface routes. Ramp meter rates are expressed in seconds per vehicle and/or in a vehicle per hour (vph). The approach was to apply a maximum rate of 500 vph / 7 seconds per vehicle and the minimum rate was 2,100 vph. At the minimum rate there is effectively no metering. Table III-2 shows the projected entrance ramp traffic volumes and the metering rates in both vehicles per hour for the sample scenario; westbound I-235, 20-year horizon, peak period. The table shows how the higher demand periods and locations shown in Table III-1, particularly along the western end of the corridor, would experience reduced metering rates during the peak period in order to preserve mainline traffic flow.
Table III-2: Ramp Meter Timings I-235 Westbound (20 Year Forecasts)
Ramp Queue Analysis: Average entrance ramp queues were calculated based on 15-minute ramp traffic volumes and the meter rate. This average queue was compared against an estimated storage distance based on the ramp meter location of 300 feet upstream of the gore to the start of the entrance ramp. Table III-3 shows the average queue lengths and illustrates that ramp metering can be effective in servicing demand while maintaining
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adequate storage on the entrance ramp. In all cases, maximum queues do not exceed available storage.
Table III-3: Ramp Meter Timings I-235 Westbound (20 Year Forecasts)
4. Evaluation of Operations Exhibit III-3 summarizes the ramp metering results for the proposed corridor for the I-235 westbound, PM peak, 20-year scenario and the I-235 AM Peak 20-year scenario westbound. During the period between 4 PM and 6:45 PM ramp metering is shown to have a positive impact on travel time, with an overall improvement of approximately 7%. The default value for ramp metering assumed in the TOPS-BC (benefit –cost analysis) program, which is based on a combination of empirical and research studies is 8%, so the study result appears to match experience elsewhere. It should be noted that this scenario is the most congested of all those considered and thus benefits would be lower in the shorter term. The AM peak chart at the bottom of the illustration shows that while ramp metering provides an improvement in travel time during part of the peak period, this improvement is smaller and covers a shorter time period than in the PM period.
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Exhibit III-3: East-West Segment - AM & PM Peak Period Travel Times (20 Year Forecast)
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Tables III-4 and III-5 summarize the changes in travel time that result from ramp metering for the AM and PM peak periods. Changes are shown for the I-235 east-west segment (west of University Avenue for both directions in the existing, 5-year and 20- year timeframes). The analysis for the north-south portion of the corridor is not shown since the initial analysis showed negligible impact on travel time. The following tables show that time savings are minimal in the existing and 5-year timeframes but become significant in the 20-year timeframe. In the 20-year timeframe savings achieved along the western portion of the corridor are significant in both the AM and PM periods. Time savings grow faster and are overall greater in all time periods in the westbound direction.
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Table III-4: East-West Segment - AM Peak Period Travel Time Improvements
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Table III-5: East-West Segment - PM Peak Period Travel Time Improvements
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Exhibits III-4 and III-5 show the same information in graphical format. Exhibits III-4 and III-5 show the travel time difference between the metered and non-metered condition on the east-west segment of I-235 for AM and PM peak periods for the existing, 5-year and 20-year horizons. As shown in the tables, significant benefits occur in the 20-year period but benefits start being realized for westbound PM peak traffic in the 5-year time period.
Exhibit III-4: East-West Segment - AM Peak Travel Times with and without Metering
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Exhibit III-5: East-West Segment - PM Peak Travel Times with and without Metering
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5. Summary of Operations Analysis The FREEVAL analysis indicated that there are not significant benefits to ramp metering on the north-south portion of the corridor, even in the 20-year timeframe. On the east- west portion however, benefits become significant over the twenty-year timeframe and some benefit occurs on the westbound section during the PM peak period even within the 5-year timeframe. These findings indicate that Iowa DOT should pursue ramp metering on the east-west stretch of I-235. Next steps include more detailed microsimulation analysis to assess impacts on upstream entrance ramp intersections and the surface street network, as well as more detailed evaluation and costing of any physical infrastructure requirements. The information generated in FREEVAL was further used to conduct a benefit/cost analysis that is documented in the third technical memorandum, titled Benefit/Cost Technical Memorandum.
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IV. RAMP METER DESIGN CRITERIA
A. Sketch Planning Spreadsheet Tool
A sketch planning spreadsheet tool was developed within Microsoft Excel involving multiple input parameters to calculate required storage and acceleration distances. Changing an input parameter would instantaneously impact multiple output results. The tool assisted in determining the effects different ramp metering timing policies had on vehicle queuing and therefore the required vehicle storage distances.
The spreadsheet tool utilized geometric information collected as part of the Existing Conditions Review Technical Memorandum including longitudinal lengths along the entrance ramp as well as cross section width information. Speeds along mainline were also utilized with regards to recommended acceleration length calculations.
B. Acceleration/Storage Length
Employing the meter timing information earlier presented, calculations were performed to calculate the queuing distances along the entrance ramp. As part of this analysis, the number of storage lanes was reviewed to determine the differences in queuing between a single storage lane and dual storage lanes. The geometric impacts of a dual storage lane strategy are discussed within the following section: Single vs. Dual Lane Operation.
From the analysis, it was determined that under existing traffic volume conditions, utilizing a stop bar set back distance of 300’ upstream of the physical gore could accommodate the average queues under a single lane storage strategy throughout the corridor. The 300’ set back distance was set globally throughout the study corridor and should be further evaluated in closer detail on a ramp by ramp basis as part of the final design process.
In addition to storage requirements, the spreadsheet tool assisted with determining the recommended acceleration distances from a stop position at the ramp meter stop bar to the point on convergence along the mainline. The longitudinal length measurements from the collected entrance ramp geometric information database were compared to the AASTHO Green Book minimum lengths of acceleration distances for entrance ramps (Table 10-3). The minimum acceleration lengths are dependent on the design speed of the freeway and initial speed along the entrance ramp. Minimum acceleration lengths were extracted from the table using the stop condition along the entrance ramp to closer replicate ramp metering conditions.
In the context of the I-235 corridor, the two following minimum acceleration lengths were used: - 55 MPH: 960 feet - 60 MPH: 1,200 feet
From review of the collected entrance ramp geometric information database of the 36 entrance ramps, and a stop bar set distance of 300’ upstream of the physical gore, a set of entrance ramps were identified that resulted in inadequate acceleration lengths when compared to the AASHTO Green Book. The 300’ distance provided adequate storage length for average queues at all 36 entrance ramps while maximizing the available
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acceleration distance. The list of identified entrance ramps along with potential mitigation notes to be evaluated further as part of the final design process is provided within Table III-1. In addition, Exhibit IV-1 provides an illustrative view of the identified entrance ramps.
Table IV-1: List of Entrance Ramps with Inadequate Acceleration Length Ramp Eastbound/Northbound Distance Note Ramp Westbound/Southbound Distance Note # Deficit # Deficit
2 SB-EB Valley West 295’ A 20 WB-SB E. Euclid 295’ A Drive Avenue 7 56th Street 195’ B 21 EB-SB E. Euclid 250’ C Avenue 8 SB-EB 42nd Street 420’ A 22 Guthrie Avenue 295’ B 11 Keosauqua Way 50’ F 25 E. 6th Street 110’ C 17 Guthrie Avenue 430’ B 30 Martin Luther King 25’ F Parkway 19 WB-NB E. Euclid 40’ F 32 42nd Street 315’ D Avenue 33 63rd Street 325’ E
NOTES: A: Not possible due to ramp to ramp spacing requirements B: Possible with additional pavement and reconfigured sign structure C: Possible, however would likely be cost prohibitive due to required bridge widening D: Possible with additional pavement and reconfigured overpass shoulder pavement E: Recent construction included auxiliary lane – Therefore adequate as is F: Minor distance deficit – Under 50’
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C. Stop Bar Placement
The location of the ramp meter stop bar should be determined on a ramp by ramp basis as part of the final design process. For the purpose of the feasibility study, the analysis included a global set back value of 300’ upstream of the physical gore location. This value should be considered a valid starting point for additional analysis as part of the final design process.
Each entrance ramp alignment is unique and the site specific characteristics play a significant role in determining the most appropriate stop bar location. The basic goal is to place the stop bar far enough down the entrance ramp to provide reasonable storage of vehicles but not so near the mainline that acceleration and merging onto the mainline becomes a concern. Items to consider while determining the most appropriate stop bar location as part of the final design process including the following:
Design hour traffic volumes Entrance ramp length (from ramp entrance to the gore point) Number of lanes controlled by the ramp meter: Single and dual lane ramp meters are common. The vertical grade: A downhill vertical grade will allow the stop bar to be placed closer to the mainline, while an uphill vertical grade will require additional space between the stop bar and the merge point. Sight distance from stop bar to the mainline: The location where vehicles stop should have adequate sight distance to the mainline to help facilitate a safe and efficient merge.
Exhibit IV-2 provides an illustrative view of the typical layout of a single lane metered entrance ramp along the I-235 corridor.
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Exhibit IV-2: Typical Metered Entrance Ramp Layout
D. Single vs. Dual Lane Operation
A key decision is whether to use single or dual lane metering strategies. Dual-lane capacity provides much greater flexibility in ramp meter operation and helps to relieve potential bottlenecks at upstream intersections or on intersecting surface streets. The decision is largely a function of physical characteristics, however; since providing additional roadway width capacity may be required for dual lane operation which can be expensive and may require right of way purchases. Where these restrictions exist, there needs to be a strong operational justification for incurring this additional cost.
The Existing Conditions Review Technical Memorandum inventoried the entrance ramp geometrics of the thirty-six entrance ramps and additional information regarding the findings can be referenced there. Section 1C-1 of the Iowa DOT Design Manual includes preferred pavement and shoulder widths for Interstate entrance ramps. The dimensions for single lane entrance ramps include a 4’ left shoulder, 16’ travel lane and a 6’ right shoulder. From an overall feasibility level review, the inventoried existing entrance ramp geometrics closely resemble the current Iowa DOT design criteria.
In other parts of the country where entrance ramp meters are used, the storage of queued vehicles behind the ramp meter is increased by a two-lane operation. In some regions what would have been a single lane entrance ramp has been converted into a striped 2-lane entrance ramp (e.g. Kansas City) and in other places the single lane entrance ramp converts to a 2-lane operation (e.g. Minnesota). The 2-lane entrance ramp configuration (whether striped or defacto operation) would allow vehicles to queue side-by-side and would help reduce the necessary longitudinal storage length. Additional information regarding the Minnesota DOT (MnDOT) and Missouri DOT (MoDOT) cross section strategies are as follows:
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Minnesota DOT Ramp Meter Criteria During ramp metering operations (typical morning and evening peak periods), MnDOT will utilize the existing single lane entrance ramp as dual lane operations. Signs are posted upstream of the ramp meters informing motorists to create two lanes of queuing. A picture of typical MnDOT single lane metered entrance ramp is included to the right (including “Form 2 Lanes When Metered” sign shown at far right).
MnDOT’s single lane retrofit (single to dual lane operation) typical cross section which utilizes metering includes a 2.5’ left shoulder, 17’ travel lane and a 2.5’ right shoulder for a total cross section width of 22’. During times of ramp metering, vehicles travel over the painted edge striping and utilize the available shoulders. Additional information from the MnDOT road design manual can be found within Appendix 2B.
Missouri DOT Ramp Meter Criteria The Missouri DOT (MoDOT) utilizes dual lane cross section widths at all times of the day. The typical Missouri DOT dual lane entrance ramp cross section provides a 6’ left shoulder, two 12’ travel lanes and a 10’ right shoulder for a total cross section width of 40’. A picture of typical MoDOT dual lane metered entrance ramp is included below.
Typical cross section illustrations of the prior mentioned state DOT criteria are included within Exhibit IV-3.
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Exhibit IV-3: Example DOT Typical Cross Sections
Due to the retrofit nature of the ramp metering implementation along the I-235 corridor, it is recommended that the MnDOT style be deployed along the study corridor as the MoDOT style would require substantial capital investments in order to widen the existing ramp entrances to dedicated two-lane cross sections.
From review of the collected entrance ramp geometric information database of the 36 entrance ramps, the average cross section width of a single lane entrance ramp including shoulders is approximately 27 feet (greater than the MnDOT typical cross section width of 22 feet) allowing the use of dual-lane operation.
In the event of an emergency, response vehicles should be able to bypass queued traffic with a cross section width of over 20 feet when a single lane of queuing is present. In the case of dual lane storage scenario, queued entrance ramp traffic would need to merge into a single lane of queued traffic allowing a bypass lane. This scenario should be discussed further as part of the final design process.
All entrance ramps included a cross section width of over 20 feet with the lowest entrance ramp having a cross section width of 21 feet (WB 73rd Street). During the design phase of the westbound 73rd Street entrance ramp metering deployment, additional consideration should be exercised to review potential mitigation strategies, if decided necessary, for the 1 foot cross section width deficit (22 feet – 21 feet).
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In general from the conducted analysis, current and short-term traffic volume demand does not require dual lane operation along the corridor, however the Iowa DOT may wish to consider the implementation of dual lane operation initially as a long term strategy in efforts to minimize future reconstruction efforts by adding additional infrastructure to provide for dual lane operations.
The decision to deploy single lane operation or dual lane operations should be made on a ramp by ramp basis as part of the final design process. As discussed within the Single vs. Dual Lane Operation section of this technical memorandum, the existing entrance ramp geometrics align with the cross section widths necessary to deploy dual lane operations and provide for emergency vehicle by-pass operations.
E. Dual Lane Vehicle Release Operations
Dual lane metered designs can release vehicles simultaneously or alternate between the two storage lanes. The simultaneous strategy releases both vehicles from each of the side by side storage lanes at the same time. Upon departing the stop bar, the vehicles must determine right-of-way amongst themselves prior to merging with mainline traffic.
With the alternating release strategy, the storage lanes alternate the green cycle indication allowing vehicles to better stagger their entry into the mainline. As part of this strategy, it is possible for each lane to operate with a different metering rate.
It is recommended that an alternating vehicle release strategy be deployed in order to stagger vehicles’ entry into the mainline. Exhibit IV-4 illustrates the two alternative vehicle release strategies.
Exhibit IV-4: Typical Vehicle Release Strategies
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F. Pavement Marking/Signing Modifications
Pavement Marking: Under either the single lane or dual lane operation strategy, significant pavement marking modifications are not anticipated other than the addition of the stop bar. If dual lane operation is to be utilized, it is not advised to stagger the stop bar and the stop bar should be marked across the shoulders in addition to the travel lane.
As part of the final design process, the physical condition of the existing longitudinal striping should be evaluated to determine if restriping is necessary.
Signing: When entrance ramps are metered, appropriate signing should be implemented along the entrance ramp as well as on nearby surface streets (if determined necessary) to alert motorists of the presence and operation of ramp meters and to the specific driving instructions they need to perform when approaching an entrance ramp. Signing needs for metered ramps also depend on the selected metering approach, and the number of available storage lanes. Typical locations of ramp metering signing is shown as part of Exhibit IV-1 and descriptions of the potential signs to be installed as part of a ramp meter are provided below. The list of requisite items below factor into the benefit/cost portion of the study.
Ramp Metered When Flashing This warning sign should be installed as part of all ramp meter installations. It should be located at the entrance to the entrance ramp and it must be visible to each legal move that enters the ramp from the adjacent side street. This may require more than one sign depending upon the entrance ramp geometry and number of lanes. This sign is intended to warn motorists prior to the motorists entering the entrance ramp as well as alert drivers to the presence and operation of ramp meters. The sign has a flashing yellow beacon attached positioned to the top of the sign panel.
Form 2 Lanes When Metered In some locations, entrance ramp shoulders or wide entrance ramps may be used during metering operations to help manage queues. This sign should be positioned near the beginning of the queue storage area on the right side (or positioned on both sides of the entrance ramp) and is used to convert the single-lane entrance ramp into a dual-lane queue storage area.
Stop Here on Red Signs that read “Stop Here on Red” should be placed on one or both sides of the entrance ramp at the stop bar to identify the stopping location. Typically, if the ramp meter only controls one lane of traffic, only one of these signs is positioned along the right side of the entrance ramp. For dual lane applications, two signs are required, one on each side of the entrance ramp. This sign helps align motorists over the demand (check-in) detectors
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placed upstream of the stop bar.
One Vehicle per Green This sign should be installed as part of all ramp meter installations, one for each lane of traffic controlled by the ramp meter. This sign is part of the ramp meter assembly. This sign is used to indicate the number of cars that are allowed to pass on each green signal.
Other Signing Other signs along the entrance ramp (i.e. merge, lane transition, etc.) will need to be taken into consideration as part of the final design process when locating the signs and meter equipment along the entrance ramp.
G. ITS Infrastructure Improvements
The following section details various communication system devices that are typically required at each of the entrance ramps where ramp metering is to be implemented using a traffic responsive system rather than a pre-timed system. A pre-timed system does not require the detection of mainline traffic flow, and is therefore not able to adapt to varying traffic flow conditions, which can lead to unnecessary queuing and requirement of continual monitoring by DOT personnel.
Each traffic responsive metered entrance ramp typically possesses an individual local controller and a varied number of vehicle detectors. Each device is described in more detail below using guidance from the FHWA Freeway Management and Operations Handbook.
Vehicle Detectors: Vehicle detectors are devices to measure traffic volume conditions along the freeway and along the entrance ramp. The vehicle detectors are used for different functions, including the following:
Check-In (Demand) Detector – When a vehicle is detected by the check-in detector, a signal is received by the ramp metering controller, which in turn will turn the signal to green, provided the red time has elapsed. In some cases, two detectors are used for this function to provide redundancy to reduce the impact of detector failures. Check-Out Detector – When a vehicle is permitted to pass the ramp metering signal, it is detected by the check-out detectors. The green interval is terminated as soon as the vehicle is sensed by the check-out detector. Queue Detector – Are commonly used to prevent the queue from spilling back and affecting the surface street operations. Detectors are placed at strategic locations towards the top of the entrance ramp. Mainline Detectors – Traffic responsive ramp metering implementations require the use of mainline detectors. Detector data across mainline lanes will be used to adjust metering rates.
Exhibit IV-5 provides an illustrative view of a typical entrance ramp and the arrangement of typical field components.
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Exhibit IV-5: Typical Entrance Ramp Metering Communications System Layout
(FHWA Freeway Management Handbook, 1997)
Local Controller: The local controller is the device which receives and stores vehicles detector information and operates the signals according to the prescribed internal logic or according to a central command system, such as a traffic management center. The controller processes data from the detector data and controls the ramp metering timing. The local controller is typically stored within an independent road-side cabinet; however, shared cabinets have been used before. Currently, the most common controller used for ramp metering operations is the Type 170 controller; however Type 2070 and ATC family controllers are becoming increasingly popular.
The controller may provide the following control functions:
Control the ramp meter signal head(s) Store and execute metering schedules Implement local traffic responsive control algorithms using mainline detector data Accept metering rate command signals from the central command system Adjust the metering rate or terminate metering to prevent entrance ramp queues from affecting cross street operations Control the advance warning sign beacon
From review of the existing communications infrastructure along the study corridor, the following additional communication devices would be required as part of the final design process for the implementation of traffic responsive ramp metering at entrance ramps along the corridor. The list of requisite items below factor into the benefit/cost portion of the study.
Dedicated vehicle detectors at each entrance ramp o Additional vehicle detectors along the mainline would also be likely Controller at each entrance ramp Communications to each controller o Backbone fiber optic communications would be preferred due to the reliability over wireless; however wireless options could be explored by piggy-backing onto existing wireless point-to-point communication protocols.
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V. SUMMARY OF FINDINGS
The Ramp Management Strategy Review component was the second installment in a series of three technical memorandums to study the feasibility of ramp metering along the I-235 corridor. The Ramp Management Strategy Review component of the study was divided into three components.
Together, the first and second components provided an overview of the overall ramp management strategy including evaluation of isolated entrance ramps versus a group strategy assessment of metering timing parameters and an operational analysis of the effects of ramp metering. The existing conditions analysis for I-235, documented in the Existing Conditions Review Technical Memorandum, demonstrated that sections along the I-235 study corridor operate at or near capacity through several interchanges along the corridor, in particular along the east-west segment. These sections do not generally experience the type of dramatic speed reductions that would justify an isolated ramp metering strategy, however aggregated together, corridor speed reductions exist.
The east-segment, from University Avenue west, has a number of deficient and potentially deficient segments, with most occurring in the westbound direction. Due to the anticipated corridor operational benefits and to minimize trip diversion to the surface street network, the group strategy along the east/west segment is recommended over the isolated strategy. The geographical extents of the east/west segment are between the western terminus, the southwest I-35/I-80 systems interchange (Southwest Mixmaster) and University Avenue (east of downtown). If resources are limited, priority should be given to the segment from 2nd Avenue west, in the westbound direction.
The north-south segment of I-235, from University Avenue north to the end of the study area at the I-35/I-80 systems interchange, has limited congestion and relatively low projected growth. The north/south segment should be monitored in the future, however near term benefits of ramp metering strategies are not currently present and even long- term benefits are projected to be negligible.
Also, due to the variability of traffic volumes temporally along the corridor both by peak period as well as seasonality, the use of a traffic responsive metering system is recommended.
The final component provided an overview of the proposed ramp meter design criteria including single lane or dual lane operation and if dual, whether a dual/simultaneous vehicle release or alternating vehicle release strategy is utilized. From use of a developed sketch planning spreadsheet tool, it was determined that in general, current and short-term traffic volume demand do not require dual lane operation along the corridor, however the Iowa DOT may wish to consider the implementation of dual lane operation as a long term strategy and minimize future reconstruction efforts by adding the additional infrastructure required for dual lane operations. The decision to deploy single lane operation or dual lane operations should be made on a ramp by ramp basis as part of the final design process. As discussed within the Single vs. Dual Lane Operation section of this technical memorandum, the existing entrance ramp geometrics generally align with the cross section widths necessary to deploy dual lane operations and provide for emergency vehicle by-pass operations. Under the dual lane operation strategy, an alternating vehicle release strategy is recommended over a dual/simultaneous release strategy.
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In addition, the final component provided an overview of the proposed ramp meter design criteria including stop bar placement, pavement marking/signing modifications and necessary ITS infrastructure improvements. With respect to the stop bar placement, the location of the ramp meter stop bar should be determined on a ramp by ramp basis as part of the final design process. For the purpose of the feasibility study, the analysis included a global set back value of 300’ upstream of the physical gore location. This value provided a reasonable balance between storage and acceleration needs along the entrance ramps and should be considered a valid starting pointing for additional analysis as part of the final design process. Pavement marking/signing modifications along with ITS infrastructure improvements associated with ramp metering implementation were also identified.
The third memorandum is titled Benefit/Cost Technical Memorandum. Based on the ramp management strategies and locations identified within this technical memorandum, a review of the anticipated user benefit and implementation/operating costs of the recommended ramp metering concept will be completed. The goal of the Benefit/Cost Technical Memorandum is to quantify the relative benefit of implementing the ramp management strategy along the I-235 corridor.
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VI. APPENDIX 2A: OPERATIONAL ANALYSIS
INCLUDED WITHIN ATTACHED DIGITAL STORAGE DEVICE
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VII. APPENDIX 2B: MnDOT DESIGN CRITERIA INFORMATION
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FEBRUARY, 2001 ROAD DESIGN MANUAL (ENGLISH) 6-2(15)
It should be noted that the configuration shown in Figure 6-2.06C (b) is technically a Type C; however, per the HCM the developed weave methodology will give only a rough approximation of capacity. Generally, this weave configuration should be avoided in cases where there is any significant ramp-to-ramp volume. Regardless of the calculations from the HCM or HCS, the length of weaving area should not be less than 1000 ft. Preferably, the interchanges should be designed to eliminate weaving. However, this will undoubtedly result in higher construction costs. The user benefits must be weighed against the additional costs to determine if the elimination of weaving is worth the expense.
CONFIGURATIONS FOR TYPE C WEAVING AREAS. Figure 6-2.06C
6-2.07 Ramp Controls 6-2.07.01 Policy On-ramps to freeway type facilities in the Metro Area may be metered. Some of the metered ramps which carry buses and other High Occupancy Vehicles (HOV) may be provided with HOV bypass lanes. Ramp meters and HOV ramp bypass lanes are the most easily implemented freeway controls. Experience in Minnesota, and various national reports and circulars have shown that ramp metering reduces the disruptive effect of congestion and helps freeways operate at higher capacity. The HOV ramp bypass lanes are implemented for the purpose of giving a priority to the multi-passenger vehicles; they encourage car pooling while increasing the persons per mile use of the freeway.
6-2.08 Metered Ramps Ramps may be metered as one lane, as two metered lanes, as two metered lanes with an HOV bypass, and as two metered lanes with a metered HOV bypass. The single lane metering applies only to retrofit situations where widening of a ramp or loop is not practical, and in some cases to new construction where the Traffic Management Center decided to implement one lane metering. In all other cases, a two lane metering of the on-ramps and loops shall be designed. All the foregoing discussion of various metered combinations is for a ramp that during the off-peak periods operates as a single lane ramp. Any two lane on-ramps which are metered, and any ramp-street junctions of metered ramps which receive double left turn movements are special cases requiring a special design.
6-2.08.01 System-to-System Metered Ramps Normally, the system-to-system ramps are not metered. But if the Traffic Management Center determines that a particularly high volume ramp or a ramp system, when metered, will have improved operation, they may request that metering be implemented. Figure 6-2.09A shows the development of the system-to-system metered ramp and the HOV bypass lane. 6-2(16) ROAD DESIGN MANUAL (ENGLISH) FEBRUARY, 2001
6-2.08.02 Operational Criteria The recommended basis for the operational criteria for metered ramps is as follows: 1. The majority of metered ramps and loops will operate as free flowing single lane ramps during the off-peak periods 2. During the metering phase of the operation, the majority of the metered ramps and loops should provide two lanes of vehicle storage up to the meter location. This is done for more efficient metering operation, maximized storage and driver expectation. 3. Generally, a six minute peak hour storage for the design hour desirably should be provided on all metered ramps. 4. The signal heads are to be placed approximately 500 ft (350 ft minimum) from the freeway-ramp junction nose. This distance, in conjunction with the acceleration lane portion, will allow most vehicles to approach mainline speeds before starting to merge with the traffic in the through lane.
6-2.08.03 Design Details Single lane ramps and loops which will operate as two lane metered facilities should preferably have the following features in their design: 1. The roadway portion of the ramp preceding the ramp meter should be 22 ft wide. This width will adequately provide for two lane metering and still allow for one lane operation in the off-peak periods. 2. Rural design ramps and loops should maintain standard width shoulders in addition to the 22 ft wide pavement. 3. A minimum of 50 ft of uniform standard 16 ft ramp width, or 18 ft in the case of widened loops, should be provided at the ramp nose when tapering out the additional metered ramp width.
6-2.09 High Occupancy Vehicle (HOV) Ramp Bypass Lanes HOV ramp bypasses give the HOV traffic at the ramp meters the advantage over single occupancy vehicles. HOV bypasses should be considered on all metered ramps. To conform to driver’s expectation, HOV bypass lanes should be developed as follows: 1. Loops – Since the majority of drivers navigate to the inside of a sharp curve, and storage is difficult to accommodate on loops, the HOV bypass should be developed to the outside (driver’s left) of the loop. With this design, the nose of the separating island will not be in the path of regular traffic. Therefore, during off peak periods, loops with HOV bypass will operate the same as loops without HOV bypass. 2. Right curving ramps with 6 deg. or sharper curves – the HOV bypass should be developed on the left side for the same reason listed in number 1 above. 3. Ramps with straight alignment, left curvature or right curvature that is flatter than 6 deg., the HOV bypass should be developed on the driver’s right (see Figures 6-2.09A and B). This places the off peak traffic closer to the freeway side, which is consistent with driver’s expectations.
6-2.09.01 HOV Bypass Design Criteria 1. The HOV bypass is designed to operate only during the time when the ramp is being metered. During off peak times, all traffic should use the main portion of the ramp. For this reason, the approach to the bypass should be designed such that a conscious effort has to be made by the driver entering the bypass, see Figure 6-2.09A. 2. A raised island up to 8 ft wide with B4 curb shall separate the HOV bypass from the main portion of the ramp. 3. Free right turns adversely affect the entrances to HOV bypasses. If practical, designers should consider eliminating the free right turn when an HOV bypass is constructed. Where double left turn lanes (to the on-ramp) and an HOV bypass are present, free right turns shall not be allowed. Figure 6- 2.09C shows an HOV bypass lane when double left turn lanes are present. 4. The TMC should be contacted for input regarding queue length and storage. 5. If the projected peak traffic storage demand is such that an overflow from the storage area will block the entrance to the bypass lane, the storage length should be increased and additional lane width, striped as a diamond lane, should be provided. See Figure 6-2.09A, note 8. 6. The minimum recommended length of a ramp is 1300 ft. This length would allow entering vehicles to approach mainline speeds. FEBRUARY, 2001 ROAD DESIGN MANUAL (ENGLISH) 6-2(17)
RAMP HOV BYPASS Figure 6-2.09A