Tight Diamond Interchange versus Single Point Urban Interchange: Pedestrians Prospective Ahmed Amer, M.S. Ph.D. Candidate Graduate Research/Teaching Assistant Department of Civil & Environmental Engineering Center for Sustainable Mobility, Virginia Tech Transportation Institute 3500 Transportation Research Plaza (0536) Blacksburg, VA 24061 Phone: (540) 357-4554 Fax: (540) 231-1555 Email: aamer@vt.edu Total word count: (text) + (tables & figures) = 2,472 + 2,500 = 4,972 Paper submitted on February 27, 2009 to the Student Paper Competition of the 2 nd International Symposium on Freeway and Tollway Operations 2009.
1 ABSTRACT This paper assesses the impact of reducing the pedestrian design speed from 4 ft/s to 3.5 ft/s as recommended by the American with Disability Act (ADA) on signalized interchanges, namely, the Single Point Urban Interchanges (SPUI) and the Tight Diamond Interchange (TDI). This study investigated the performance of the two types with different traffic loadings and configurations in terms of the control delay and using computer simulation. The results of the study show that the SPUI provides a significant improvement in the efficiency of the interchange over a TDI in medium and heavy traffic loadings, due to the use of a single signal and free flowing right turn movements. It was also found that either configuration provide comparable results under light traffic volumes. The traffic distribution had a small effect on control delay, while reducing pedestrian speed has the greatest impact when used with heavy traffic volumes rather than light and medium volumes. INTRODUCTION There have been many arguments over the use of the Tight Diamond Interchanges (TDI), over the Single Point Urban Interchanges (SPUI). Generally, the SPUI is similar to the TDI with one major difference; that the ramp terminals of the SPUI are joined into one crossing that is operated with one signal. The SPUI is characterized by its ability to allow concurrent off-ramp left-turns to proceed simultaneously, by compressing the two intersections of a diamond into one single intersection over or under the free-flowing road. On the other hand, The TDI is characterized by two closely spaced signalized intersections where the ramps terminate at the cross street and are served with the crossroad movements. Due to the close proximity of the two signals, the signal operations of the two intersections are operated together as one signal. The general layout configuration of each type is shown in Figure 1. Tight Diamond Interchange (TDI) Single Point Urban Interchange (SPUI) Figure 1: General Layouts of the TDI and the SPUI
2 There are many advantages for the TDI and the SPUI over each other. The TDI is pretty much easier than the SPUI for both; the road user to understand and the traffic engineer to design and operate. The problem with the SPUI is that the single intersection is geometrically very wide which makes the driver feels lost inside the intersection. Regarding design and operation, because the SPUI compresses the two simple signals of the TDI into one complicated signal, which makes it more challenging to design and operate. Alternatively, the SPUI is generally associated with fewer delays, especially with heavier traffic volumes. Apparently, the SPUI seems to be operationally more recommended rather than the TDI. Nevertheless, if the pedestrians are to be introduced into the signal design and operation, the delay associated with the SPUI might be longer than that with the TDI. This may be attributed to the longer clearance time needed for the SPUI wide intersection, compared to the two small intersections of the TDI. Hence, the use of TDI versus the SPUI is still a question. Furthermore, there are calls by the American with Disability Act (ADA) for adapting the pedestrians design requirements to take care of the disabled pedestrians. The traditional design guidelines by MUTCD recommends walking speed of 4 ft/s [1], whereas the ADA Accessibility Guidelines (ADAAG) recommends a reduced speed of 3.5 ft/s [2]. Such reduction in the pedestrian speed implies longer clearance times for the pedestrians at the intersection, which magnifies the debate of using the TDI or the SPUI. This paper aims to compare the performance of the TDI versus the SPUI corresponding to including the pedestrians in the design of signal timing. In addition, the paper investigates the effect of using the ADAAG walking speed (3.5 ft/s) instead of the MUTCD speed (4 ft/s). These comparisons are to be made in terms of the control delay, which is considered as the main measure for the intersection level of service. Control delay is the delay experienced by a driver due to congestion at an intersection due to deceleration while approaching the intersection, stopping in a queue at the intersection, and acceleration after the queue is released. Rather than the pedestrian speed, many other parameters are involved in the comparison, for instance, the total traffic volume, the balance of the traffic between the main arterial and the freeway ramps. BACKGROUND The SPUIs are used as alternatives to using a typical diamond interchanges in areas with constrained right-of-way. There is a single signalized intersection, servicing ramps and crossroad traffic. A three phase signal control is commonly used with a leading left-turn and through movement for cross street traffic [3]. The first phase controls both crossroad left turn movements. In the second phase both crossroad and through movements take place. Finally, in the third phase both off ramp left turn movements are made [4]. A yield condition is in place for right-turn ramp movements [5]. Using the SPUI configuration can provide 30 to 50 percent better efficiency than the TDI. For the TDI to perform better than a SPUI the following three conditions would likely need to be observed: increasing split of the cross-street thought volumes, increase of the volume of the cross-street left turn opposing the heavy through movement, and imbalanced off-ramp left turns. Under most traffic conditions the SPUI configuration provides better capacity than a diamond interchange. At lower V/C ratios, SPUIs still provide a lower average delay [6]. When choosing between the SPUI and the TDI, traffic volumes are a very important parameter to look at. California recommends the use of a SPUI when the AADT for the major highway through movement is between 20,000 and 35,000 vehicles or the minor road volumes is
3 between 15,000 and 30,000 vehicles. A TTI report states that the SPUI should be used with the total AADT for entering traffic is 40,000 vehicles [7]. TTI created a SPUI delay model utilizing the sum of critical flows as the independent variable. Two models were developed, one for no right turn on red and one for a yield right turn condition. Their results showed that at a sum of critical flow value of approximately 0.5 there is a difference in vehicle delay of 10 seconds from each of the models, with the yield condition having a lower value. The separation in delay then increases exponentially as the ratio also increases [7]. Furthermore, presence of pedestrians at signalized intersections necessitates combined pedestrian-vehicle phasing intervals. A pedestrian interval is provided to discharge pedestrian traffic safely. Furthermore, walking speed is a crucial factor in the design of a pedestrian change interval because it determines how much time a pedestrian actually is given to cross the road. In general, the pedestrian interval includes a WALK interval and a flashing DON T WALK (clearance) interval. According to the Manual on Uniform Traffic Control Devices (MUTCD) [1], the WALK interval gives pedestrians adequate time to perceive the WALK indication and depart the curb before the clearance interval begins. It is recommended to be 7 seconds at least. On the other hand, the clearance interval time allows pedestrian crossing to leave the curb and travel with walk speed to at least the far side of the traveled way or to a median of sufficient width for pedestrians to wait before opposing vehicles receive a green indication. The MUTCD recommends walk speed of 4.0 ft/sec. However, the Americans with Disability Act (ADA) resulted recently in several calls to lower the walk speed to 3.5 ft/sec [2]. Accordingly, the total pedestrian interval is calculated as where, d t = D+ u t is the minimum pedestrian green time (seconds) D is the WALK interval time (seconds) d is the crossing width (feet) u is the walk speed (ft/sec) Nevertheless, introducing the pedestrians in the design of the signal timing and its effect on the performance of the TDI and the SPUI was not covered heavily in the literature. Not only, this paper introduces the pedestrians into the comparison, but also it compares the impact of reducing the existing design walking speed (4 ft/s) to a recommended reduced speed (3.5 ft/s). In 2002, the ADAAG suggested a walking speed of 3 ft/s [9] to be used instead of the MUTCD traditional speed of 4 ft/s [1]. Nevertheless and based on many comments (e.g. AASHTO comments [10]), the ADAAG revised their report with a maximum design walking speed of 3.5 ft/s [2]. STUDY METHODOLOGY This study uses computer simulation to measure the average control delay per vehicle in the TDI and in the SPUI corresponding to including the pedestrians in the signal timing. Two pedestrian walking speeds are used; MUTCD speed (4 ft/s) and ADAAG speed (3.5 ft/s). Three different traffic loadings were used to evaluate the interchanges; light (4000 vph), medium (5500 vph), and heavy (7000 vph). The volumes were chosen by researching the number of vehicles serviced for various levels of service in previous studies. Two traffic distributions between the arterial and the freeway ramps were tested; a balanced distribution (70% on main arterial and
4 30% on freeway ramps) and heavy ramp distribution (60% on main arterial and 40% on freeway ramps) were used with the three traffic volumes. The combinations of these different cases give a total of 24 traffic loading scenarios as summarized in Figure 2. Those should model the most encountered situations in the field. Balanced (70/30) Balanced (70/30) MUTCD Speed (4 ft/s) MUTCD Speed (4 ft/s) Heavy Ramp (60/40) Heavy Ramp (60/40) TDI VS SPUI Balanced (70/30) Balanced (70/30) ADAAG Speed (3.5 ft/s) ADAAG Speed (3.5 ft/s) Heavy Ramp (60/40) Heavy Ramp (60/40) Figure 2: Summary of the 24 Loading Scenarios To simulate these 24 traffic scenarios, the total hourly volume for each scenario was distributed into specific movements. These movements were then coded into Synchro [11] to obtain optimized signal timings. Traffic volumes, movement distributions, and signal timings were then entered into CORSIM [12]. Each traffic scenario was modeled and observed using TRAFVU. Any problems in the configuration of the simulation could be detected in this phase and modified to correctly model the desired condition. It is worth mentioning here that signal timing in the cases of the TDI was developed to minimize the number of vehicles stored between the two signals. Simulations were run to determine the best conditions for each type of interchange. The geometric configuration of the TDI and the SPUI used in the simulation included two through lanes in each direction for cross street traffic, dual left-turn lanes on the cross street and from the ramp terminals, a right turn lane on the off-ramps, and a right turn lane for cross street to on-ramp movements. The geometric layout of the TDI and the SPUI are shown in Figure 3.
5 1 2 TDI SPUI Figure 3: Geometric Layout of the TDI and the SPUI
6 Traffic Volumes The two traffic distributions that were studied were balanced and heavy ramp situations. The balanced distribution placed 70% of the total traffic on the main arterial and 30% on the ramp movements. The ramp heavy distribution shifted 10% of the arterial movement to the ramps, giving the main arterial 60% of the total flow and the ramps 40%. Three traffic volumes were used within the two distributions. The volumes were selected by studying previous research and observing the range of traffic by which a SPUI would be beneficial. The previous studies gave mixed results using this range of loading. The light loading used an hourly volume of 4000 vehicles, the medium loading used an hourly volume of 5500 vehicles, and the heavy loading used an hourly volume of 7000 vehicles. The individual movement volumes can be found in Table 1. Table 1: Traffic Volumes of Individual Movements (vph) Movement Balanced Distribution Heavy Ramp Distribution Light Medium Heavy Light Medium Heavy Total 4000 5500 7000 4000 5500 7000 EBR 200 275 350 171 236 300 EBT 800 1100 1400 686 943 1200 EBL 400 550 700 343 471 600 WBR 200 275 350 171 236 300 WBT 800 1100 1400 686 943 1200 WBL 400 550 700 343 471 600 NBL 400 550 700 533 733 933 NBR 200 275 350 267 367 467 SBL 400 550 700 533 733 933 SBR 200 275 350 267 367 467 Signal Timing Initial signal timings were calculated by using Synchro based on a pretimed 3 phase control. The traffic volumes and distributions were entered along with interchange geometry to optimize the signal timings. For the TDI, the default values for yellow and all red times were used; yellow of 3 s and all red of 1 s. For the SPUI configuration, the yellow and all red times were increased by one second each to accommodate for the longer clearance; yellow of 4 s and all red of 2 s. Thereafter, the minimum split was calculated based on the pedestrian speeds (4 ft/s or 3.5 ft/s). The NEMA phasing and the optimized signal timings for the TDI two signals and for the SPUI single signal are given in Table 2 and 3, respectively.
7 Table 2: NEMA Phasing and Optimized Timing for the TDI Signals Pedestrian Distribution Volume Intersection # Cycle (sec) Offset (sec) Green Time (sec) Phase 1 Phase 2 Phase 3 Int.2 Int.1 Yellow (sec) All Red (sec) MUTCD ADAAG Heavy Ramp Balanced Heavy Ramp Balanced Light Medium Heavy Light Medium Heavy Light Medium Heavy Light Medium Heavy 1 76 62 30 14 20 3 1 2 76 0 14 20 30 3 1 1 93 78 31 21 29 3 1 2 93 0 21 29 31 3 1 1 106 90 30 26 38 3 1 2 106 0 26 38 30 3 1 1 72 58 30 13 17 3 1 2 72 0 13 17 30 3 1 1 90 72 32 19 27 3 1 2 90 0 19 27 32 3 1 1 93 78 32 19 30 3 1 2 93 0 19 30 32 3 1 1 84 69 34 16 22 3 1 2 84 0 16 22 34 3 1 1 100 84 34 22 32 3 1 2 100 0 22 32 34 3 1 1 118 102 34 29 43 3 1 2 118 0 29 43 34 3 1 1 78 64 34 14 18 3 1 2 78 0 14 18 34 3 1 1 100 80 35 23 30 3 1 2 100 0 23 30 35 3 1 1 100 80 35 21 32 3 1 2 100 0 21 32 35 3 1
8 Table 3: NEMA Phasing and Optimized Timing for the SPUI Signal Pedestrians Distribution Volume Cycle (sec) Green (sec) Phase 1 Phase 2 Phase 3 Yellow (sec) All Red (sec) MUTCD ADAAG HRmp Blncd HRmp Blncd Light 68 10 19 21 4 2 Medium 82 14 29 21 4 2 Heavy 101 20 42 21 4 2 Light 62 9 14 21 4 2 Medium 70 10 21 21 4 2 Heavy 85 14 30 23 4 2 Light 65 10 13 24 4 2 Medium 80 12 25 25 4 2 Heavy 100 19 39 24 4 2 Light 65 10 13 24 4 2 Medium 75 10 22 25 4 2 Heavy 90 15 31 26 4 2 CONTROL DELAY DATA/ANALYSIS CORSIM was used to run the simulations for the 24 scenarios and to compile the control delay data for each. The traffic volumes and signal timings were used to calculate control delay for each link. The different links of the TDI and the SPUI are classified as illustrated in Figure 4. Nevertheless, to better analyze and describe the control delay, it is to be analyzed by each movement. Such delay was calculated by adding the delay of each link in the movement. This gave a better indication of how each vehicle would be affected by its destination. TDI Link Classification Figure 4: TDI and SPUI Link Classification SPUI Link Classification
9 Balanced Traffic Distribution Scenarios The control delays for the balanced traffic distribution scenarios using both pedestrian speeds are summarized by movement in Table 4. The analysis shows that the total control delay associated with the TDI is higher than that associated with the SPUI in almost all traffic volume cases, especially in the medium traffic volume. This is attributed to that the pedestrians increased the signal split for the minor movements; freeway ramps. Furthermore, the application of the ADAAG speed almost has slight effect on all balanced scenarios for the two interchanges, except for the heavy traffic with the TDI which was increased one and half times. Table 4: Summary of Control Delay (sec) for Balanced Traffic MUTCD Speed Movement Light Medium Heavy TDI SPUI TDI SPUI TDI SPUI EBT 49.1 22.3 209.2 33.5 188.1 134.4 EBR 24.6 0.8 176.3 6.2 160.1 102.3 EBL 49.1 22.3 209.2 33.5 188.1 134.4 WBT 33.5 21.5 200.8 35.9 187.8 128.0 WBR 17.3 0.8 177.8 5.7 162.0 97.2 WBL 33.5 21.5 200.8 35.9 187.8 128.0 NBR 13.0 0.3 19.2 0.7 30.7 1.2 NBL 13.0 11.6 19.2 21.1 30.7 30.1 SBR 9.0 0.2 16.6 0.7 57.3 1.2 SBL 9.0 14.0 16.6 18.6 57.3 29.1 Total 251.1 115.3 1245.7 191.8 1249.9 785.9 ADAAG Speed Movement Light Medium Heavy TDI SPUI TDI SPUI TDI SPUI EBT 47.2 38.7 209.2 97.0 205.7 137.4 EBR 27.0 1.9 173.4 59.3 171.1 105.1 EBL 47.2 38.7 209.2 97.0 205.7 137.4 WBT 39.9 34.3 197.5 91.0 193.8 127.4 WBR 19.0 1.2 175.8 53.2 168.8 94.7 WBL 39.9 34.3 197.5 91.0 193.8 127.4 NBR 13.5 0.4 19.5 0.6 33.8 1.0 NBL 13.5 10.9 19.5 18.4 33.8 26.9 SBR 10.4 0.3 18.9 0.7 272.1 1.1 SBL 10.4 10.6 18.9 16.6 272.1 28.0 Total 268.0 171.3 1239.4 524.8 1750.7 786.4
10 Heavy Ramp Traffic Distribution Scenarios The control delays for the heavy ramp traffic distribution scenarios using both pedestrian speeds are summarized by movement in Tables 5. The analysis shows that the total control delay associated with the TDI is still higher than that associated with the SPUI in all traffic volume cases. The effect of the reduced ADAAG speed almost has slight effect on all balanced scenarios for the two interchanges, except for the medium traffic volumes. This is due to increasing the delay for large number of vehicles which increases the overall control delay. Table 5: Summary of Control Delay (sec) for Heavy Ramp Traffic MUTCD Speed Movement Light Medium Heavy TDI SPUI TDI SPUI TDI SPUI EBT 39.3 21.4 86.1 25.5 246.4 120.7 EBR 19.6 0.7 53.1 1.2 199.0 88.3 EBL 39.3 21.4 86.1 25.5 246.4 120.7 WBT 31.3 20.4 214.7 25.9 236.1 92.9 WBR 14.8 0.7 188.5 1.3 198.3 59.1 WBL 31.3 20.4 214.7 25.9 236.1 92.9 NBR 11.5 0.4 20.4 1.0 215.1 1.8 NBL 11.5 11.9 20.4 14.4 215.1 23.4 SBR 8.3 0.4 32.0 1.0 263.5 2.0 SBL 8.3 11.3 32.0 15.5 263.5 20.0 Total 215.2 109.0 948.0 137.2 2319.5 621.8 ADAAG Speed Movement Light Medium Heavy TDI SPUI TDI SPUI TDI SPUI EBT 46.4 25.9 154.4 37.3 253.9 150.3 EBR 24.1 0.8 119.9 5.3 212.4 113.6 EBL 46.4 25.9 154.4 37.3 253.9 150.3 WBT 32.3 24.1 203.5 29.3 232.5 155.8 WBR 16.9 0.7 177.0 1.7 203.1 120.3 WBL 32.3 24.1 203.5 29.3 232.5 155.8 NBR 11.0 0.5 20.6 1.0 117.1 1.9 NBL 11.0 9.7 20.6 14.3 117.1 21.2 SBR 8.6 0.4 69.4 1.0 233.4 2.0 SBL 8.6 11.5 69.4 16.2 233.4 23.0 Total 237.6 123.6 1192.7 172.7 2089.3 894.2
11 SUMMARY AND CONCLUSIONS The results of this paper show that the SPUI was much more efficient than the TDI under almost all traffic loadings and different pedestrian speeds. On all simulations, the SPUI had significantly shorter control delay. The total control delay of the 24 scenarios is summarized in Table 6. Table 6: Total Control Delay (sec) for the 24 Scenario Total Control Delay (sec) Balanced Heavy Ramp MUTCD ADAAG MUTCD ADAAG Low Medium Heavy TDI 251.1 268 215.2 237.6 SPUI 115.3 171.3 109 123.6 TDI 1245.7 1239.4 948 1192.7 SPUI 191.8 524.8 137.2 172.7 TDI 1249.9 1750.7 2319.5 2089.3 SPUI 785.9 786.4 621.8 894.2 The results of this study show that the SPUI provides a significant improvement in the efficiency of the interchange over a TDI in heavy traffic loadings. This is due to the use of a single signal and free flowing right turn movements. There is still a benefit to using a SPUI in medium traffic loadings, but the TDI is easier for the user and the designer. Either configuration could be chosen for light and medium traffic volumes. Traffic distribution had a small effect on control delay. Furthermore, the reduction of the pedestrian walking speed has minor effect on the control delay with the light and the medium traffic volumes. Nevertheless, the ADAAG walking speed is not highly recommended in the areas associated with heavy traffic volumes, either with the TDI or with the SPUI. ACKNOWLEDGMENT The author is more than appreciative for Dr. Montasir Abbas, for his enlightening guidance to the author while conducting this research. Thanks also go for Dr. Hesham Rakha for sponsoring the author during his research. In addition, the author would like to acknowledge Christ Harris, M.S., for his contributions to this work. Gratitude also is due to Hossam Hablas, M.S., who gave an appreciated assistance to the analysis made in the paper.
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