Intersection of Massachusetts Avenue and Maple Street in Lexington Signalized Intersection and Roundabout Comparison Michael Wallwork, Roundabout Expert, Orange Park, Florida Tom Bertulis (MS, PE, PTOE), Design Consultants Inc., Somerville, Massachusetts December 2, 2015 SIDRA Analyses Summary with notes for consideration Overall, a low-speed, one-lane roundabout with a right turn lane on each leg under congested conditions is expected to provide superior operation to a signalized intersection with the same number of entering lanes. Analysis 1 compares the performance of various roundabout layouts compared to the proposed signalized intersection that also has two approach lanes on each leg. Analysis 1 Common factors used for roundabout analyses 1. PHF = 1.00 2. 35 mph approach speed 3. 2% trucks 4. Traffic volumes provided Design Consultants, Inc. (peak flows of 3,200 3,500 vehicles/hour) 5. Environmental Factor = 1.00 6. LOS same as signalized intersections Common factors used for signalized intersection analyses 1. PHF = 1.00 2. 35 mph approach speed 3. 2% trucks 4. Traffic volumes provided Design Consultants, Inc. (peak flows of 3,200 3,500 vehicles/hour) 5. Minimal pedestrian phases at signals 6. Maximum signal cycle time of 90 seconds to provide efficient operation by balancing delay and vehicle queues One lane roundabout with three (3) entry lanes One lane roundabout with one RT, (4) entry lanes One lane roundabout with NB and WB RT (5) entry lanes 1
One lane roundabout with RT, SB LT, (5) entry lanes One lane roundabout with 2 RT and LT (6) entry lanes Signals with six (6) entry lanes Table 1 SIDRA analyses using fixed 2.9-second gap and 2.0-second headway. As explained below, these key parameters are derived from the SIDRA Default analysis in Table 2, field studies, and our engineering judgment to provide the most accurate prediction of roundabout performance. Configuration Peak Period Level-of- Service Average Delay (sec) Average Queue (ft.) 95 th Percentile Queue (ft.) Volume/cap acity ratio AM B 14.6 301 748 WB 0.988 PM D 40.2 724 1,800 WB 1.213 with WB RT lane with WB + NB RT lanes with SB LT and WB RT lanes with SB LT and WB + NB RT lanes Signalized intersection with three turn lanes AM A 4.8 250 623 SB 0.945 PM B 19.6 654 1,403 SB 1.064 AM A 4.6 247 614 SB 0.943 PM B 18.7 553 1,375 SB 1.061 AM A 9.8 111 277 SB 0.794 PM B 16.6 181 451 SB 0.894 AM A 2.0 110 275 SB 0.793 PM B 5.1 179 445 SB 0.892 AM D 35.7 679 1,109 SB 0.949 PM F 121.6 1,472 2,403 SB 1.205 2
Table 1 above uses SIDRA standard (not HCM) with a gap of 2.9 seconds with 2.0-second headway, which is more representative of drivers at low-speed roundabouts in congested conditions. In these circumstances most peak-hour drivers are commuters using the same roundabout twice a day who, over a short learning period, can be found to be using even smaller gaps. In off-peak periods, when vehicle flows are smaller, there is a greater variety of drivers gap sizes that increase into the 3 to 4 second range, similar to the standard SIDRA defaults used in Table 2 below. A field study carried out by DCI Inc. at the nearby Washington Street/Blanchard Road/Grove Street roundabout in Belmont found an even lower average critical gap: 2.6 seconds during saturation flow rate conditions in the PM peak period. Analysis 2 Table 2 below uses SIDRA defaults, that is, where SIDRA sets the gap and headway sizes based on the level of congestion. These parameters (see Appendix A) were used to confirm the choice of fixed gap and headway in Analysis 1 above. In particular, in some cases SIDRA Defaults used similar gap sizes and smaller headway settings, which increase the vehicle entry rate into a roundabout. Table 2 Analyses using SIDRA Default values Configuration Peak Period Level-of- Service Average Delay (sec) Average Queue (ft.) 95 th Percentile Queue (ft.) Volume/capa city ratio AM E 70.7 906 2,252 WB 1.369 PM F 118.5 1,348 3,352 WB 1.663 with WB RT lane with WB + NB RT lanes with SB LT and WB RT lanes with SB LT and WB + NB RT lanes Signalized intersection with three turn lanes AM D 35.2 819 2,035 SB 1.158 PM E 64.5 1,257 3,125 SB 1.297 AM C 32.4 790 1,963 SB 1.147 PM E 60.0 1,236 3,073 SB 1.289 AM A 3.7 148 368 NB 0.847 PM B 10.3 248 618 NB 0.935 AM A 2.4 121 302 SB 0.795 PM A 5.5 191 474 SB 0.889 AM D 35.7 679 1,109 SB 0.949 PM F 121.6 1,472 2,403 SB 1.205 Over the years the SIDRA Analysis Program has had a number of changes made to it to bring it more in line with the original study of roundabout capacity. As a result, gap sizes in SIDRA outputs have increased compared to earlier models. A recent study found that the original study capacity figures were quite low: 1,130 vehicles per hour per lane as the maximum flow per lane, whereas the recent study has increased that figure to 1,430 entering vehicles per lane, a whopping 26.5 percent increase in saturation 3
flow that is not yet incorporated into SIDRA. SIDRA will be upgraded to the new figures once the new saturation values have been formally adopted in the US, in case of any changes during the review period. In this case, the gap sizes varied from 2.92 to 4.35 seconds and headways from 1.92 to 2.55 seconds. In each case, the sizes of the gaps and headways varied little because the congestion levels are very similar. See gap and headway values in Appendix A. After SIDRA is upgraded to the new saturation values, these are expected to fall closer to the values used in Table 1 (2.9 sec gap and 2.0 sec headway). Summary Overall, a low-speed, one-lane roundabout with a right turn on Maple Street, a southbound left turn lane and a northbound right turn lane on Massachusetts Avenue under congested conditions is expected to provide optimum performance. However, all eight possible combinations of 0, 1, 2, or 3 bypass lanes can be considered in order to meet geometric and other constraints, and all are expected to provide superior operation to a signalized intersection. Analysis 1 compares the performance of various roundabout layouts to the proposed signalized intersection that also has two approach lanes on each leg. Notes 1. Level-of-service and average delay is given for the overall intersection, which can result in average delay smaller than level-of-service because of extreme variations in vehicle flow in one of the three legs such as that shown in item two in Table 2 above, where two legs have very good performance and the third extremely poor performance. It is necessary to review the output summary sheets for this situation in greater detail. 2. Gap acceptance sizes are important in that they have a significant impact on performance. Under very congested conditions at low-speed roundabouts acceptable gaps are small. As speeds increase acceptable gap sizes increase. Roundabouts designed for low-speed operation, less than 23 mph design speeds, typically operate at low speeds, around 9 to 16 mph. There results are actually from a detailed study of the Clearwater Beach roundabout, a two-lane roundabout with design speeds varying from 9 to 16 in peak periods and 15 to 23 mph in off-peak periods. Therefore, smaller gaps are acceptable under such circumstances. 3. Roundabouts are vastly safer for all users, especially pedestrians. Also, at roundabouts pedestrians do not have to ask permission to cross the road (push a button); they have the right-of-way and can cross roundabouts almost at will, but with due care, and have little impact on roundabout capacity because pedestrians typically walk though gaps in the traffic flow. 4. The above is the peak-hour performance only. In the off-peak hours the roundabout has superior performance when compared to signalized intersections, especially at night and on weekends. Other Issues 1. Traffic volumes vary from day to day by upwards of 10 percent but we seek very precise analyses. 2. If traffic volumes increase over time, delays and queues will degrade, but the degradation with traffic signals will be even greater. 3. Capacity is only one issue to consider; pedestrian safety and mobility are often even more important these days, and capacity can be less important than intersection safety. 4. Signalization is not one of the nine safety improvements promoted by the Federal Highway Administration, but roundabouts are. 5. Roundabouts also reduce noise, fuel consumption and pollution levels. 4
One-lane roundabout AM Peak APPENDIX A Gap Sizes and Headway Values from Analysis 2 One-lane roundabout PM Peak 5
One-lane roundabout AM Peak with Westbound Right Turn Lane One-lane roundabout PM Peak with Westbound Right Turn Lane 6
One-lane roundabout AM Peak with Westbound and Northbound Right Turn Lanes One-lane roundabout PM Peak with Westbound and Northbound Right Turn Lanes 7
One-lane roundabout AM Peak with Southbound Left Turn Lane and Westbound and Northbound Right Turn Lanes One-lane roundabout PM Peak with Southbound Left Turn Lane and Westbound and Northbound Right Turn Lanes 8