Efficiency and Safety Evaluation of Unconventional Intersections
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Archive of SID 8th National Congress on Civil Engineering, 7-8 May 2014 Babol Noshirvani University of Technology, Babol, Iran Efficiency and Safety Evaluation of Unconventional Intersections Pooya Najaf1, Srinivas S. Pulugurtha2 1- Research and Teaching Assistant, INES Ph.D. Candidate, The University of North Carolina at Charlotte, NC, USA 2- Associate Professor and Graduate Program Director, Department of Civil & Environmental Engineering, The University of North Carolina at Charlotte, NC, USA 1- [email protected] 2- [email protected] Abstract The conventional countermeasures such as pre-timed, semi-actuated, and actuated signals, signal coordination systems, multiple left-turn lanes, and intelligent transportation systems are not able to overcome the safety and operational problems of traffic congestion (e.g., delay, fuel consumption, crashes, and pollution) in many urban and suburban areas. The unconventional designs have therefore attracted engineers’ attention as an alternative to solve these problems. Two main objectives of the unconventional designs are reducing delay and the number of conflict points. The unconventional alternatives may reduce the number of conflict points by re-routing some left-turns. It is, however, vital to know whether they can improve safety by reducing the number and intensity of traffic conflicts between vehicles. This research focuses on evaluating the efficiency and estimating the number and intensity of conflicts between vehicles (as a surrogate safety measure) for six most common unconventional intersection (unsignalized median U-turn, signalized median U-turn, super street, bowtie, forward jughandle, and reverse jughandle) and three conventional intersection (pre-timed, optimized, and actuated) designs by simulating the real- world traffic information using VISSIM, Synchro and Surrogate Safety Assessment Model (SSAM) software. According to the final results, pre-timed conventional intersection has the worst performance, while unsignalized and signalized median U-turn intersections have the best performance (i.e., lowest average delay, highest average speed, and lowest number of conflicts, except their high conflicts’ intensity and their long travelled distances). Furthermore, forward and reverse jughandles are safest designs. In addition, super streets cannot be appropriate design alternatives due to their low average speed, high average delay, and high number of conflicts. Keywords: Traffic Simulation, Unconventional Intersections, Traffic Conflicts, Measures of Effectiveness 1. INTRODUCTION As the growth rate of population and auto ownership is higher than construction of new roadways and transportation facilities, traffic congestion is a major problem in many urban and suburban areas. Congestion leads to several operational, environmental, and safety problems as well as increase in fuel consumption. Since transportation engineers have not overcome these problems using demand management policies and other conventional countermeasures, unconventional designs have gained their attention [1]. Two main principles of unconventional alternatives are reducing delay to through vehicles and the number of conflict points at intersections [2]. However, it is still important to study how much they can reduce the delay and what is their effect on the number and intensity of traffic conflicts. Six types of unconventional intersections are studied in this research, including unsignalized median U-turn, signalized median U-turn, super street, bowtie, forward and reverse jughandle. Their effectiveness is compared after simulation in VISSIM [3] using the real-world information. The number and intensity of conflicts between vehicles is also studied using Surrogate Safety Assessment Model (SSAM) software [4]. Comparing these unconventional designs with the basic conventional intersections including pre-timed, optimized (simulated in Synchro [5]), and actuated (simulated in VISSIM) signals helps to 1 www.SID.ir Archive of SID 8th National Congress on Civil Engineering, 7-8 May 2014 Babol Noshirvani University of Technology, Babol, Iran understand how effective and safe are the different types of unconventional intersections. A brief description of aforementioned unconventional designs is presented next. a) Unsignalized and Signalized Median U-Turn In the case of unsignalized and signalized median U-turn, the left-turns to and from the arterial use median crossovers (Figure 1-a). This unconventional design helps eliminate left-turns, while a combination of right-turn and U-turn replaces each left-turn [2]. b) Super Street The super street is an extension of the median U-turn in which through arterial movements have higher priority. As it is shown in the Figure 1-b, the four intersection approaches are designed as two independent three- approach intersections [2]. c) Bowtie The bowtie (Figure 1-c) is another type of the median U-turn design with the median and directional crossovers on the cross-street. This eliminates left-turn at the main intersection. The bowties could be roundabouts on the cross-street. This overcomes the disadvantage of requiring a wide right-of-way (ROW) on the cross-street [2]. d) Jughandle The forward jughandle (Figure 1-d) uses ramps prior to the intersection. This design helps to diverge the left- turns from the right side of the arterial [2]. Reverse jughandle (Figure 1-e), on the other hand, uses roundabouts for arterial left-turns. Major Major Minor Minor Figure 1-b- Super Street Figure 1-a- Median U-turn Major Major Minor Minor Figure 1-c- Bowtie Figure 1-d- Forward Jughandle Major Minor Figure 1-e- Reverse Jughandle Figure 1- Different Types of Unconventional Designs 2 www.SID.ir Archive of SID 8th National Congress on Civil Engineering, 7-8 May 2014 Babol Noshirvani University of Technology, Babol, Iran Some of the advantages and disadvantages of the aforementioned designs is as follows [2, 6, 7 and 8]. The advantages of the median U-turn intersection include reduced delay for through arterial movements, increased capacity, easier progression for through arterial traffic and fewer stops, fewer threats to pedestrians, and fewer and more separated conflict points. Its disadvantages include driver confusion, increased delay for left-turns, increased travel distances and stops for left-turns, and need for larger ROW. The advantages of the super street over a conventional intersection include reduced delay for through arterial movements and also for left-turns from the arterial, reduced stops for through arterial traffic, fewer threats to pedestrians, and reduced and separated conflict points. Its disadvantages include driver confusion, pedestrian confusion, and increased delay, travel distances and stops for cross-street movements and also for left-turns to the arterial. The advantages of the bowtie over the conventional signalized intersections include reduced delay for through arterial movements, increased capacity, reduced stops and easier progression for through arterial movements, fewer threats to pedestrians, and reduced and more separated conflict points. Its disadvantages include driver confusion, possible driver disregard for the left-turn prohibition, increased delay, travel distance and number of stops involving left-turn traffic and possibly cross-street through traffic, and confusing arterial U-turn. The advantages of the jughandle over the conventional signalized intersections include reduced delay and stops and easier progression for through arterial movements, narrower ROW, and reduced and more separated conflict points. Its disadvantages include driver confusion, increased delay, travel distances and stops for left-turns from the arterial, more difficult movements for pedestrians to cross ramps and the main intersection, and lack of access to arterial for parcels next to ramps. 2. DATA DESCRIPTION Studied corridor, in this research, is extracted from Moon et al. [1] paper, in which the number of conflicts and average travel time is compared between a real conventional intersection and a simulated super street. The study area is an arterial in a rural area on the four-lane divided National Highway 38, Gyeonggi-do, South Korea. The corridor section with a length of 1,670 meters and speed limit of 80 km/h contains three signalized intersections, spaced by a distance of about 400 meters (Figure 2). In this research, all vehicles including all traffic movements on the corridor are studied (i.e., network performance) rather than comparison between through major arterial and minor cross-street traffic flows. The values in parenthesis, in the figure, show the percentage of heavy vehicles and yellow time for each phase is 3 seconds. Based on the real field data, the signalized intersections are uncoordinated and with different pre-timed cycle lengths. The figure shows 15 min peak hour (18:00-18:15) turning volumes for each intersection along with the proportion of heavy vehicles for the corridor and signal phases [1]. 363 m 375 m Minor Street 1 Minor Street 2 Minor Street 3 1 (0) 4 (25) 6 (0) 6 (0) 74 (35) 6 (0) 7 (14) 6 (10) 10 (0) 89 (10) 95 (8) 22 (4) 121 (5) 141 (3) 87 (0) 145 (3 9 (0) 4 (0) 10 (0) 13 (0) 203 (5) 199 (5) 233 (8) 153 (14) 153 (0) 175 (13) 6 (17) 10 (0) Phase Phase 14 (0) 3 (0) Phase Time 120 24 Time 17 90 25 Time 13 22 12 30 Figure 2- Study Area [1] Since the real volumes are not balanced, in this research new balanced volume values are used in the simulation procedures (Figure 3). 3 www.SID.ir Archive of SID 8th National Congress on Civil Engineering, 7-8 May 2014 Babol Noshirvani University of Technology, Babol, Iran 1 6 6 6 74 22 4 11 10 4 6 9 12 238 121 204 152 203 110 145 139 178 153 154 6 175 13 10 14 Figure 3- Balanced Volumes 3 3. SIMULATION PROCESS a) Simulating the Conventional Intersection First, the aforementioned corridor with the balanced volumes, same geometric characteristics, and existing pre-timed signalization is simulated in VISSIM. b) Calibration Using Field Data Calibration is needed after simulating the main network in the VISSIM. This step is strongly dependent on the availability of the data. Traffic volumes, travel time, delay, and average speed are most common parameters for calibration.