A Comparison of a Roundabout to Two-way Stop Controlled

A Comparison of a Roundabout to Two-way Stop Controlled Intersections with Low and High Traffic Volumes Greg Luttrell1, Eugene R. Russell2 and Margaret Rys3

Abstract In 1997, the City of Manhattan installed the first roundabout in the state of Kansas. A Kansas State University research team conducted a field study of the operation of the roundabout. Comparison to similar traffic flows at stop controlled intersections were made. Using the measured field conditions as a starting point, the SIDRA computer model was used to evaluate a range of traffic conditions. SIDRA was used to replicate field conditions to test the roundabout and comparable two-way stop intersection operation at increasing traffic levels. The results of the roundabout and comparable intersection operations are compared. The comparison yields information usable by other jurisdictions when evaluating traffic control options. The methodology should be valuable for any community desiring to consider roundabouts as a viable, cost-effective intersection traffic control device. Introduction This study examined the operation of a roundabout compared to two-way stop intersection control. Traffic counts were observed by way of a specially designed 360o view video camera. The camera was linked to video recording equipment. The collected traffic counts became inputs into the computer evaluation program: Signalized and Unsignalized Intersection Design and Research Aid, version 5.20b (SIDRA). SIDRA is able to evaluate all types of intersections including roundabout and two-way stop. SIDRA is based on gap acceptance theory adopted by the Australian Road Authority [Troutbeck, 1993] and similar to that adopted for use in the Highway Capacity Manual for roundabout analysis. Six of the SIDRA analysis outputs were selected as measures of effectiveness (MOEs) for comparison of intersection operation; 1) 95% queue, 2) average delay, 3) maximum approach delay, 4) proportion stopped, 5) maximum approach stopped, and 6. degree of saturation. These MOEs were statistically analyzed to determine how the roundabout and two two-way stop controlled intersections compared to one another. The Manhattan Roundabout The Manhattan roundabout was the first modern roundabout built in the state of Kansas. It is located at the intersection of two collector roads adjacent to a residential area. For three years prior to 1997, the intersection had an average of three crashes per year with at least one involving injury. Citizens were concerned and wanted a change in traffic control. The City Engineer decided to install a roundabout, which was completed in the fall of 1997. The Manhattan roundabout controls a four-leg intersection. All legs are two lanes (one entering and one exiting) and parking is allowed on both sides of all approach roads. All approaches are YIELD controlled, have raised splitter islands and marked crosswalks (see Figure 1). There is one 4.6 m (15 feet) circulating lane around a 12.8 m (42 foot) diameter central island. The approach lane width ranges from 4.0 to 4.9 meters (13 to 16 feet).

Figure 1 - Manhattan Roundabout Hourly Traffic Volumes Jacquemart summarized existing US roundabouts in relation to hourly entering traffic volumes. The hourly volumes ranged from around 4700 vehicles (Long Beach) to 300 vehicles (Las Vegas) [Jacquemart, 1998]. The Manhattan roundabout carries traffic at the bottom end of the range for existing roundabouts in the United States. Twenty-two hourly entering volumes recorded for study analysis ranged from 224 to 402 vehicles. The average hourly entering volume for the Manhattan roundabout was 310 vehicles. Comparable Two-way Stop Intersections Two comparable two-way stop controlled intersections were also studied. Both were at the intersection of collector roads in Manhattan residential areas. The intersections were Dickens Avenue and Wreath Avenue (DW), and Juliette Avenue and Pierre Street (JP). Hourly entering traffic at these stop intersections ranged from 215 to 480 vehicles at DW and 206 to 495 vehicles at JP. The average hourly entering volumes were 340 and 328 vehicles respectively. Statistical testing indicated that all hourly samples were normally distributed and could be considered to be from similar populations. This was important in that the statistical analysis was based on an assumption of having comparable normally distributed data. The results presented here are for the low and high traffic levels of 380 and 570 vehicles/ hour. All statistical testing was performed on mean values from each MOE at each intersection.

Data Collection Each vehicle must be tracked through a roundabout to determine what turn was made. Traditional traffic counting techniques were not considered feasible for this study, as resources were not adequate. The City of Manhattan obtained and installed a specially designed video camera and recording equipment for data collection. The camera was supplied by Intelligent Highway Systems, Inc. [List, 1997] and provided a full 360o view of each intersection. Videotape records allowed the data to be collected and viewed later for examination. The camera was installed on an existing street light pole at the intersection (see Figure 2). Video images were fed into a VCR assembly inside a weather tight cabinet. The camera was mounted at a height of approximately 6.5 meters (20 feet) which provided ample sight of the entire intersection. Once the videotapes were collected, they were viewed and traffic counts recorded in 15-minute intervals. This allowed the heaviest traffic flows from each tape to be used for analysis. SIDRA Analysis The data was input into the computer program SIDRA to obtain values for the six MOEs selected for the analysis of the three intersections (see Table 1). The intersection geometric features required for SIDRA were based on measurements taken from the construction plans and/or field measurements.

Figure 2 - Omnidirectional Video Camera Mounted on Street Light Pole

Table 1 - Intersection Measures of Effectiveness Measure of Effectiveness 95% Queue

Description Total length of the queues for all approaches at the 95% confidence levela Average Delay Average vehicle delay for all entering vehicles (seconds/ vehicle) Maximum Approach Delay Average vehicle delay for the approach with the highest average vehicle delay (seconds/ vehicle) Proportion Stopped Proportion of entering vehicles that are required to stop due to vehicles already in the intersection Maximum Proportion Proportion of entering vehicles that are required to stop Stopped due to vehicles already in the intersection on the approach with the highest proportion stopped value Degree of Saturation Amount of capacity that is consumed by the current traffic loading (referred to as v/c ratio) a Queue lengths were based on a vehicle length of 8 meters (25 feet) SIDRA evaluates the operation of an intersection using the maximum hourly flow as calculated by the equation: VolSIDRA = Volhour / Peak Hour Factor. Therefore, the SIDRA analysis hour volumes are higher than the actual hourly volumes counted. The peak hours were those that occurred during each of the study hours. Statistical Analysis The output from SIDRA was analyzed to determine how the operation of the roundabout compared to the two comparable two-way stop controlled intersections. Statistical tests were performed using the Statistical Analysis Software, version 6.12 (SAS), on the Kansas State University, Unix operating system. Two base assumptions exist for the use of most statistical tests: normality and equal variances. These two data assumptions were tested prior to determining what specific statistical tests to use to evaluate the intersection operation. Once normality and variance determinations were made, appropriate statistical methods were employed to determine differences in the data sets. 95% Queue The 95% queue values ranged as shown in Table 2. The data was found to be normally distributed with equal variances. Data groupings were found to be CG ≠ DW ≠ JP. Based on the mean values, the 95th percentile queue at the roundabout lies between the two-way stop intersections at both the low and high traffic levels (DW < CG < JP).

Table 2 - 95% Queue Intersection Roundabout Two-way Two-way (CG) Stop (DW) Stop (JP) 7.9 m (26 ft) 3.7 m (12 ft) 8.5 m (28 ft) Low Traffic Level 16.5 m (54 ft) 13.1 m (43 ft) 25.9 m (85 ft) High Traffic Level 11.0 m (37 ft) 7.0 m (24 ft) 16.0 m (52 ft) Mean (Xbar) 2.4 m (7.9 ft) 3.0 m (9.7 ft) 4.7 m (15.2 ft) Standard Deviation (S)

Average Delay The average intersection delay values are shown in Table 3. The average delay values were found to be normally distributed with unequal variances. The statistical tests indicated that the roundabout experienced higher average delays at both traffic levels (DW < JP < CG). Table 3 - Average Vehicle Delay Intersection Roundabout (CG) 7.8 sec/ veh Low Traffic Level 7.9 sec/ veh High Traffic Level 7.9 sec/ veh Mean (Xbar) 0.160 sec Standard Deviation (S)

Two-way Stop (DW) 3.3 sec/ veh 4.5 sec/ veh 4.0 sec/ veh 0.544 sec

Two-way Stop (JP) 3.5 sec/ veh 6.2 sec/ veh 4.7 sec/ veh 0.826 sec

Maximum Approach Delay The maximum approach delay values are shown in Table 4. These values were found to be normally distributed with unequal variances. The statistical tests indicated that the roundabout was operated with the lowest maximum average vehicle delay for both traffic levels (CG < DW < JP). Table 4 - Maximum Approach Average Vehicle Delay Intersection Roundabout (CG) 8.4 sec/ veh Low Traffic Level 9.2 sec/ veh High Traffic Level 8.9 sec/ veh Mean (Xbar) 0.237 sec Standard Deviation (S)

Two-way Stop (DW) 9.6 sec/ veh 9.9 sec/ veh 9.5 sec/ veh 0.823 sec

Two-way Stop (JP) 10.1 sec/ veh 12.2 sec/ veh 10.9 sec/ veh 0.734 sec

Vehicle Delay The difference in the average vehicle delay values stems from the way in which a roundabout operates. By design, roundabouts equalize delays to all approaches. Two-way stop controlled intersections provide priority to one street and penalize the other. This priority setting works well if the free flow street has the major traffic flow. This was not always the case at Juliette Avenue and Pierre Street. Therefore, there is a disparity in the delays on the approaches at two-way stop intersections that does not exist in a roundabout.

Proportion Stopped The ‘proportion stopped’ is a measure provided in SIDRA and represents the proportion of vehicles stopped by other traffic already in the intersection. The values for proportion stopped can range from 0.0 to 1.0 (see Table 5). The proportion stopped values were found to be normally distributed with unequal variances. The roundabout was found to experience a level of proportion stopped between the other two intersections (DW < CG < JP). Table 5 - Proportion Stopped Intersection Roundabout (CG) 0.15 Low Traffic Level 0.22 High Traffic Level 0.18 Mean (Xbar) 0.0236 Standard Deviation (S)

Two-way Stop (DW) 0.11 0.19 0.15 0.0267

Two-way Stop (JP) 0.21 0.34 0.27 0.0466

Maximum Proportion Stopped The maximum proportion stopped values are those that occur on the approach experiencing the highest proportion stopped value (see Table 6). The maximum proportion stopped values were found to be not normally distributed. The non-parametric analysis indicated the roundabout operated with the lowest value for this MOE (CG < JP < DW). Table 6 - Maximum Approach Proportion Stopped Intersection Roundabout (CG) 0.21 Low Traffic Level 0.30 High Traffic Level 0.25 Mean (Xbar) 0.0276 Standard Deviation (S)

Two-way Stop (DW) 0.36 0.49 0.41 0.0646

Two-way Stop (JP) 0.32 0.50 0.38 0.0751

Degree of Saturation The degree of saturation (volume to capacity (v/c) ratio) values (see Table 7) were found to be not normally distributed. The Kruskal-Wallis test found the distributions to be different. The roundabout had a slightly higher v/c ratio at the low traffic level (380 vehicles/ hour) and a lower v/c ratio at the high traffic level (570 vehicles/ hour). Examining the means found the roundabout to operate better than the other two intersections with respect to v/c ratio (CG < DW < JP).

Table 7 - Degree of Saturation Intersection Roundabout (CG) 0.080 Low Traffic Level 0.139 High Traffic Level 0.095 Mean (Xbar) 0.0231 Standard Deviation (S)

Two-way Stop (DW) 0.053 0.174 0.106 0.0510

Two-way Stop (JP) 0.058 0.224 0.136 0.0500

Study Summary This study evaluated the operation of an existing roundabout located in Manhattan, Kansas. This study evaluated the operation of the roundabout in Manhattan, Kansas versus two comparable two-way stop intersections. Intersection operation was evaluated using six measures of effectiveness (MOEs). A summary table of the results of the MOE evaluation is shown in Table 8. No conclusions were drawn for the MOEs of 95th percentile queue and proportion stopped. The roundabout operated with a higher average delay, but with lower maximum approach delays than the comparable two-way stop intersections. The roundabout was also found to experience a lower amount of vehicles stopped by intersection traffic on the approach experiencing the highest amount of stoppage. Table 8 - Summary of MOE Statistical Results Measure of Effectiveness Statistical Result Operational Advantagea 95% Queue DW < CG < JP Not determined Average Delay DW < JP < CG Two-way stop Maximum Approach Delay CG < DW < JP Roundabout Proportion Stopped DW < CG < JP Not determined Maximum Approach Stopped CG < JP < DW Roundabout Degree of Saturation CG < DW < JP Roundabout a based on having the best performance on each MOE. All statistical testing yielded results at the 95% confidence level.

Safety In the three years prior to roundabout construction, the intersection experienced an average of three crashes a year with at least one involving injuries. The average annual cost of these crashes was found to be $87,833 [Russell, 2000]. In the 2½ years since the roundabout has been in operation, there have been no reported crashes. This has been determined to be a statistically significant reduction in crashes and cost savings. Conclusions Under three of the four MOE conditions where conclusions were able to be determined, the roundabout was found to operate statistically better than the comparable two-way stop intersections. There have been no reported crashes in the 2 ½ years since the roundabout was

completed representing a statistically significant reduction. The decision to build this roundabout was a good one. The conclusions from this study may apply to other locals only if the overall conditions are similar to that found in this study. Acknowledgements Funding for this study was supplied by the Mac-Blackwell Transportation Center with matching funds from the Kansas Department of Transportation and an in-kind contribution and assistance, from the City of Manhattan. The authors are solely responsible for all statements and conclusions. References • Jacquemart, G., Modern Roundabout Practice in the United States, Synthesis of Highway Practice 264, Washington, D.C., Transportation Research Board, National Research Council, 1998 • List, G., and Waldenmaier, E., Omnidirectional Video Instrumentation for Unsignalized Intersections and Roundabouts, Third International Symposium on Intersections Without Traffic Signals, Portland, OR, 1997 • Russell, G., Rys, M., and Luttrell, G., Modeling Traffic Flows & Conflicts at Roundabouts, Mac-Blackwell National Rural Transportation Study Center, University of Arkansas, Fayetteville, AR, 2000 • Troutbeck, R., The Guide to Traffic Engineering Practice Roundabouts, Austroads, Sidney, Australia, 1993 Author Information 1. Greg Luttrell, P.E., Graduate Research Assistant, Kansas State University, Manhattan, KS, member of ITE 2. Dr. Eugene R. Russell, P.E., Civil Engineering, Kansas State University, Manhattan, KS, fellow of ITE 3. Dr. Margaret Rys, Industrial and Manufacturing Systems Engineering, Kansas State University, Manhattan, KS