Evaluation of Roundabout Safety

Transcription

1 Evaluation of Roundabout Safety Xiao Qin, Ph.D., P.E. (Corresponding Author) Assistant Professor Department of Civil and Environmental Engineering, South Dakota State University CEH, Box, Brookings, SD, 00 Phone: (0) - Fax: (0) - Xiao.qin@sdstate.edu Andrea Bill Associate Researcher Traffic Operations and Safety (TOPS) Laboratory, University of Wisconsin-Madison Department of Civil and Environmental Engineering Engineering Drive Madison, WI 0 Phone: (0) - bill@wisc.edu Madhav Chitturi, Ph.D. Research Associate Traffic Operations and Safety (TOPS) Laboratory, University of Wisconsin-Madison Department of Civil and Environmental Engineering Engineering Drive Madison, WI 0 Phone: (0) 0- mchitturi@wisc.edu David A. Noyce, Ph.D., P.E. Professor Director, Traffic Operations and Safety (TOPS) Laboratory, University of Wisconsin-Madison Department of Civil and Environmental Engineering 0 Engineering Hall Engineering Drive Madison, WI 0 Phone: (0) - noyce@engr.wisc.edu Word count: 0+ ( Tables and Figure)*0=,0 Submitted for Presentation and Publication to the nd Annual meeting of the Transportation Research Board January -, 0 Washington, DC TRB 0 Annual Meeting

2 0 ABSTRACT While roundabouts are still fairly new in the United States and Wisconsin, their safety benefits have been studied with varied results. In this study, roundabouts built in 00 or before were analyzed for their safety performance. Three years of before and after crash data were gathered as well as geometric and traffic volume data. An empirical Bayes (EB) analysis was used to examine the safety benefits for total crashes and injury (K, A, B, C) crashes. The EB analysis was performed using the Safety Performance Functions from the Highway Safety Manual. Mixed results were found for total crash frequency but a significant decrease in crash severity was identified. Wisconsin roundabouts had an unbiased estimate of a. percent decrease in total crashes. National numbers similarly show decreases in total crashes. Wisconsin roundabouts showed a significant percent decrease in injury crashes. Roundabouts nationwide are also experiencing a significant decrease in severe crashes. When evaluating predictors, the speed limit of the approaches did not show a significant impact on the safety of the roundabout. While multi-lane roundabouts seemed to be safer than single lane roundabouts when considering fatal and injury crashes, single lane roundabouts saw the largest decrease in total crashes. Two-way stop controlled intersection conversion to a roundabout had the highest safety benefit compared to all-way stop controlled and signalized intersections. TRB 0 Annual Meeting

3 Qin, Bill, Chitturi, and Noyce INTRODUCTION In a recently published Federal Highway Administration (FHWA) memo, roundabouts are considered as one of the nine proven safety improvements (). The memo is based on extensive safety research studies conducted overseas and at various locations in the United States (U.S.). Though the history of roundabout in the U.S. is relatively short compared with Great Britain and other European counties, roundabouts have demonstrated a trend in reducing crashes, especially in reducing severe injury crashes. In, Maycock and Hall developed the United Kingdom (U.K.) model to quantify the safety performance of four-leg roundabouts (). A recent study of roundabouts in the U.S. identified crash reductions of approximately percent for all crash types and percent for fatal and injury crashes when an intersection was converted from a signal or stop control to a roundabout (). The reason behind the large improvement in safety records at these locations lies in the design features of roundabouts that reduce conflict points and vehicular speeds, as well as increased driving attention. All vehicles circulate counterclockwise around a raised central island at a relatively low speed. The entering vehicles yield to vehicles already in the roundabout, thus reducing all left-turning related crashes such as head-on or angle crashes which often result in serious injuries. Lowered travel speeds also reduce the collision impact, thus reducing the crash consequence. Other design features that help to improve safety or facilitate safe movements are detailed in the FHWA Roundabout Information Guide (). Since the first modern roundabout was constructed, many safety evaluation studies have been conducted to quantitatively assess the safety benefits of this new intersection control strategy. The studies range from observational before-and-after to meta-analysis. However, these studies frequently show considerable differences in roundabout safety performance (,, ). Many factors can contribute to this disparity, and can be generally grouped into three categories: ) driving population, ) site choice, and ) evaluation methodologies. Though roundabouts may be safer by design than other intersection control strategies, however the safety benefits may be compromised by driver behavior. Inappropriate signage and pavement marking can also compromise the safety benefits. Roundabouts demand a high level of driver compliance to the traffic signs and judgment towards traffic conditions such as reducing their speed when approaching the roundabouts, judging a safe gap correctly, and yielding to the vehicles already in the roundabouts. Because roundabout entries are not always straight or perpendicular to other approaches but curved, for the drivers who are not familiar with roundabouts, proper lane choice may be challenging especially if appropriate lane assignments are not provided. The wrong lane choice often contributes to same direction sideswipe crashes in the circulatory lanes. If the design of roundabout fails to consider particular user groups (pedestrians, bicyclists, visually impaired users, etc.) and special vehicle types (large trucks), safety may also be jeopardized if these populations are prevalent (, ). Daniels, et al. found that the variation in crash rates are mainly driven by the traffic exposure as well as vulnerable road users, who are more frequently involved in crashes at roundabouts than expected based on a sample of 0 roundabouts in Flanders, Belgium (). Site choice may also be critical because some roundabouts are constructed due to operational benefits of increasing capacity, reducing delay, improving flow continuity, environmental considerations, and others. For these roundabouts, safety benefits may not be as apparent. Data collection design and methodology selection are also important for the studies conducted at different times and from different areas. TRB 0 Annual Meeting

4 Qin, Bill, Chitturi, and Noyce The research is motivated by the need for a thorough and comprehensive observational before-and-after roundabout safety evaluation in Wisconsin. The first roundabout in Wisconsin was built and opened to traffic in. Currently, there are approximately 00 roundabouts on the state trunk network with more being planned by the end of the 0 construction season. The objectives of this study are to use unbiased, comprehensive evaluation methodologies to quantify the safety of roundabouts of various conditions, and to support informed decisionmaking. LITERATURE REVIEW A number of research studies have shown that roundabouts are successful in reducing both crash frequency and severity. Robinson reported a reduction between percent and percent in the number of injury crashes in Australia, percent and percent in France, percent and percent in the U.K., and percent in the U.S. (). Retting reported a percent reduction in injury crashes in the U.S. (0). A study in seven U.S. states: Colorado, Florida, Kansas, Maine, Maryland, South Carolina, and Vermont, where a total of intersections were converted to roundabouts, used state-of-the-art Empirical Bayes (EB) before-and-after studies (). The results revealed a 0 percent reduction in all crash severities and an overall reduction of 0 percent in injury crashes. The recently published Highway Safety Manual (HSM) includes the potential crash effects of converting a signalized intersection or a stop-controlled intersection into a modern roundabout by multiplying crash modification factors (CMFs) (). Even though the CMFs vary in numbers considerably, the implications are self-evident in that roundabouts can significantly reduce crash severity and frequency. A detailed review of the literature looked at the effects of roundabouts on reduction of specific crash severity. Research in Belgium showed a percent reduction in injury crashes with substantial differences related to speed limits (). Other studies report that reduction in crash severity is more prominent for higher speed than lower speed intersections, or at an intersection without a traffic signal (). Results presented by Persaud et al. showed a statistically significant reduction of 0 percent for all crash severities combined and an 0 percent decrease for all injury crashes based on a total of intersection locations. (). In a meta-regression analysis using studies, Elvik found slightly smaller safety effects when roundabouts replaced previously signalized intersections instead of intersections controlled by yield signs ( percent instead of percent for fatal and percent instead of percent for serious injury crashes) (). The report concluded that roundabouts are clearly effective in reducing injuries and fatal crashes, but high uncertainties were found for property damage only crashes. A crucial point is that Elvik recognized that only out of studies used before-andafter studies with EB methodology while the rest used the simple before-and-after or cross sectional studies. Roundabouts also change the type of crashes and manner of collisions, which is an important indicator of the consequence. Mandavilli conducted a thorough analysis of crashes occurring at roundabouts in Maryland by classifying crashes by movement and roundabout location (). Four types of crashes were found to be a significant portion of the total: run-off-road, rear-end at entry, entering-circulating, and sideswipe same direction (in circulation). The observations were consistent with other studies even though the order of the crash type varied (,, ). A recent study in Wisconsin showed that although crash severity decreased significantly at roundabout locations, the results were mixed for crash frequency (). TRB 0 Annual Meeting

5 Qin, Bill, Chitturi, and Noyce Although most of the evidence suggests that roundabouts provide clear safety benefits, in terms of reduction in crash frequency and severity, there are several issues in regards to the safety of the road users who are vulnerable and/or visually impaired. There have been very few crash-based safety studies of those specific road users at roundabouts because of sample size issues due to the low number of motor vehicle collisions involving such users. Especially for pedestrians with sensory or mobility impairments, pedestrian behavior and safety at roundabouts are not well understood. A study by Ashmead analyzed actual street crossings by sighted and visually impaired pedestrians at a two-lane urban roundabout under high and low traffic conditions (). The research showed that visually impaired pedestrians took three times longer to cross than sighted participants. Some crossing required interventions because of danger to the participants. Wall found that crossings became increasingly difficult as vehicular volume increased and multilane roundabouts were more challenging than single-lane roundabouts in ensuring safe pedestrian access (). On the other hand, Harkey and Carter studied the interactions between motorist and pedestrians at roundabouts as part of an NCHRP project and reported no substantial safety problems for non-motorists at roundabouts based on conflicts or collisions (0). The most definitive guide on roundabouts in the U.S. has been the NCHRP Report, which explicitly mentions the need for continuing research to gain further understanding of roundabout safety (). The NCHRP Report conducted an extensive review of the safety and operational aspects of roundabouts in the U.S. based on available nationwide data and recommended various crash prediction models for different roundabout characteristics, the specifics of which can be found in the report (). The preceding studies clearly demonstrate a reduction in crashes at locations where roundabouts were constructed. However, the large variability in the results suggests that safety improvements are location-specific and may be affected by the unique situations of each location. Therefore, there is a need to further study the safety impacts of roundabouts as new data becomes available. While successful evaluations and positive results have been reported in other states, it is unclear how these can be translated to the roundabouts in Wisconsin. It has also been acknowledged that the design and operations of a roundabout may have varying safety impacts on different road users. METHODOLOGY The simple before-and-after comparison assumes that changes can be solely attributed to the safety improvement with everything else remaining unchanged before and after the improvement, which is often not true. The observational difference between before and after periods may be biased as a result of regression-to-the-mean, a phenomenon where measures of the data over a long period drift towards the mean value. A simple before-and-after analysis may inflate the countermeasure effectiveness because locations slated for safety treatments usually have high crash counts, rates, or severities. Hauer suggested using the expected number of crashes that would have occurred in the after period had the countermeasure not been implemented as the before observation. The expected mean can be formulated as the weighted average of a predicted number of crashes and site-specific crash history as follows (). E=W μ+(-w) N () where: TRB 0 Annual Meeting

6 Qin, Bill, Chitturi, and Noyce E = Expected Crash Count (Estimate of Long Term Mean over Y years) Weight of Prediction N = Observed Crashes (over Y years) μ = Predicted Number of Crashes (SPF Calculated Value for Y years) Y = Number of Years in Study k = Overdispersion Parameter The methodology of estimating the expected number of crashes is called the EB analysis and the before-and-after comparison using the expected number of crashes is called EB before-and-after analysis. Note that in the actual calculation, any change in the traffic volume (AADT) or length of the analysis time period needs to be adjusted. An adjustment factor is shown in Equation. () Multiplying the r factor to the EB expected number of crashes offers a correct estimate of the number of crashes that would have happened during the after time period had the treatment not been implemented. The safety performance can be computed for individual roundabouts. When each roundabout shows varying performance, the difference or the ratio of the total number of crashes before and after the roundabout construction can provide a quantifiable mean (average) safety performance measure as well as the variance of the measurement from an overall perspective. The ratio or odds ratio of before and after crashes, also called index of effectiveness is formulated as in Equation (): all sites all sites N N observed,a expected,a If the value of is smaller than, fewer crashes are observed than expected and the safety treatment is positive in reducing crashes. Otherwise, the safety effect is negative, which means the treatment is not effective in reducing crashes. An approximate unbiased estimate is given by Equation (): where Var Nexpected, A ri N B w, N expected, i B all sites all sites expected, A Var all sites N allsites expcted,a Similar to, if the value of is smaller than, fewer crashes occur than expected. Safety effectiveness can be calculated as 00(-). A positive percentage indicates a net percentage reduction in crashes. () () TRB 0 Annual Meeting

7 Qin, Bill, Chitturi, and Noyce The confidence intervals suggest the statistical significance of the estimated safety performance. The magnitude of the variance can be used to assist in managing the risk of investment. The variance of the unbiased estimate of in Equation measures the estimate precision. Var Nexpected, A all sites Nobserved,A N expcted,a allsites Var( ) () Var Nexpected, A all sites N expcted,a allsites SE( ) Var( ) A simpler safety effectiveness measurement is the difference between expected before N N. A positive value means that observed and after crashes given by expected, A observed,a all sites number of crashes is fewer than expected, suggesting a net crash reduction. The variance of is given by Equation : Var( ) Var Nexpected, A Var Nobserved, A Var Nexpected, A Nobserved, A all sites all sites all sites all sites () SE ( ) Var( ) DATA COLLECTION AND PROCESSING In order to perform a meaningful before-and-after comparison, a minimum of three years of data are required. All 0 roundabouts under Wisconsin Department of Transportation (WisDOT) oversight built in 00 or before were included in the study. Figure shows the locations of the thirty roundabouts included in the study. Crash Data Crash data for the roundabout locations were retrieved from the WisTransPortal from January, to December, 00 (). Relevant crash information was gathered based on the time when a crash occurred and the study area defined for a particular roundabout. Three-year before and after crash data were collected for each roundabout. Crashes that occurred during the construction year were excluded. Not limited to the intersection junction or circulatory area, the data collection allows for crashes occurring on roundabout approaches due to speeding or sudden stop or slowing down to be included. A manual review of Wisconsin crash report forms TRB 0 Annual Meeting

8 Qin, Bill, Chitturi, and Noyce (MV000) was also conducted for all queried crash data using police narratives and diagrams to differentiate whether or not crashes were truly roundabout crashes or related to roundabouts. 0 FIGURE Locations of roundabouts used for this study. Crashes occurring during the construction year were excluded from the study to minimize the effects of construction activities and other situations such as being partially open to traffic during the construction. All the roundabouts studied had a three-year before period and a threeyear after period of crash history to obtain a statistically reliable result. Six roundabouts were omitted due to either a lack of post-construction data or unique geometry, specifically: four roundabouts were newly constructed intersections and had no historic crashes; one roundabout combined several closely spaced intersections; and one roundabout had significant changes during the study period, occurring until 00. Therefore, the analysis and results presented are for roundabouts. Geometric and Traffic Data Important roundabout design features include the number of approaches, speed limit, number of circulating lanes, lane width, inscribed circle diameter (ICD), center island diameter (CID), and TRB 0 Annual Meeting

9 Qin, Bill, Chitturi, and Noyce the total AADT. The AADT at a roundabout was defined as the sum of AADT on each approach entering the roundabout. Traffic volume information was primarily collected from the Wisconsin Highway Traffic Volume Data which is published by WisDOT annually (). For the roundabouts with missing AADT, individual traffic counts were conducted. In general, researchers observed in the dataset that three-legged roundabouts carried less traffic than four-legged roundabouts. The three-legged roundabouts had an AADT range of,0 to,00 vehicles per day (vpd) with an average of,00 vpd, while the four-legged roundabouts had a range of,00 to,00 vpd with an average of, vpd. Similarly, single-lane roundabouts had lower traffic volumes than multi-lane roundabouts. In the roundabouts observed for this research study, the AADT for the single-lane roundabouts ranged from,000 to,00 vpd with an average of, vpd. For the multi-lane roundabouts, AADT ranged from,00 to,00 vpd with an average of 0,0 vpd. In addition to current AADT levels, the intersection configuration and traffic data before roundabout conversion were collected, including AADT, number of intersection approach legs, number of major roadway lanes, existence of major roadway median, speed limit, and more importantly, the traffic control type before the roundabout conversion. WisDOT Region and area type were also collected. The area type was recorded as urban if the municipality where the roundabout was located had a population greater than,000. The characteristics of the roundabouts are listed in Table. TABLE Characteristics of Wisconsin Modern Roundabouts Characteristics Number Percentage Area Type Urban Rural.%.% Number of circulating lanes Previous intersection traffic control No control/yield (NC) Two-way Stop Controlled (TWSC) All-way Stop Controlled (AWSC) Signalized RESULTS EB Analysis with HSM Safety Performance Function (SPF) 0.0% 0.0% % 0% % % SPFs can be found for a variety of highway facilities and intersection types in HSM and its companying software package SafetyAnalyst (SA) (). Appropriate SPFs were identified using the pre-roundabout intersection geometric characteristics (number of legs, number of lanes), area setting (urban, rural), as well as traffic control types (Yield, TWSC, AWSC, Signalized). SPFs can be developed for different types of crashes based on the purpose of the evaluation. The standard SPF is formulated as the equation of expected annual crash count in Equation : TRB 0 Annual Meeting

10 Qin, Bill, Chitturi, and Noyce Coefficients a, b, c, and an overdispersion factor k can be estimated via maximum likelihood estimation (MLE). Table lists the HSM or SA sources for total crashes and fatal and injury crashes by intersection type (). TABLE Intersection Type UrbYD Intersection Type Description Total Crashes Fatal and Injury Crashes RurST -lane rural -leg intersections with HSM 0- SA. minor-road stop control RurST -lane rural -leg intersections with HSM 0- SA. minor-road stop control UrbST -lane urban -leg intersections with HSM -0 HSM -0 minor-road stop control UrbSTALL -lane urban -leg intersections with allway stop control SA. SA. UrbYD -lane urban -leg HSM -0 intersections with (URBST) minor-road stop control HSM -0 (URBST) RurSG -lane rural -leg signalized intersections HSM - HSM - RurST -lane rural -leg intersections with HSM - HSM - minor-road stop control UrbST -lane urban -leg intersections with HSM -0 HSM -0 minor-road stop control UrbSTALL -lane urban -leg intersections with allway SA. stop control UrbSG -lane urban -leg signalized intersections HSM -0 HSM -0 UrbST -lane urban -leg intersections with HSM -0 HSM -0 minor-road stop control UrbSTALL -lane urban -leg intersections with all- SA. SA. way stop control -lane urban -leg intersections with minor-road stop control HSM -0 (URBST) HSM -0 (URBST) () TRB 0 Annual Meeting

11 Qin, Bill, Chitturi, and Noyce To facilitate the EB analysis, procedures were coded to a spreadsheet with all the key parameters. Tables and show the results for total number of crashes and injury (K, A, B, and C) crashes, respectively. The first two columns are intersection IDs and descriptions; followed by observed crashes during the three-year before period, the expected number of crashes during the three-year after period, and the observed number of crashes during the three-year after period. The last two columns are before-and-after comparison results with negative values suggesting an increase in crashes after building the roundabout. A few findings are highlighted from the EB results for the roundabout in Tables and.. Mixed results for changes in crash frequency: locations ( percent) experienced a decrease in crash frequency while locations ( percent) experienced an increase. On average, the increase was. crashes per year and the decrease was. crashes per year. Wisconsin roundabouts had a. percent reduction in all crash types compared to a. percent reduction observed in the national study ().. Significant crash severity reduction: locations ( percent) had a reduction in fatal (K) and injury (A, B, and C) crashes. Seven locations did not have any fatal and injury crashes after the roundabout conversion compared with only three locations before the roundabout conversion. The average reduction in fatal and injury crashes was 0. crashes per year. Wisconsin roundabouts had a reduction of percent in injury crashes compared to a. percent reduction reported in the national study ().. A detailed review of the locations that have experienced an increase in all crashes or injury crashes reveals that the majority of the sites have only incurred slight crash increases. A small percentage of roundabouts experienced significant increases in crash frequency and severity and contribute substantially to the summary statistics. The roundabout safety effectiveness can be measured using Equation to Equation. The index of roundabout effectiveness θ is 0.0, meaning less number of crashes was observed than expected and the safety treatment is positive in reducing crashes. Also, the approximate unbiased estimate ε is 0.0, meaning a. percent reduction in crashes. A simpler safety effectiveness measurement is the difference between expected before and after crashes. The value of is -. for all sites with a standard deviation of., suggesting a net crash reduction. In summary, Most of the roundabouts have shown promising safety improvements in all crash severity levels. Though mixed results were observed in total crash frequency after the roundabout conversion, a small portion ( percent) of the locations accounted for a significant portion ( percent) of the total increase. In particular, two sites STH/ at Broadway and Canal Street at th Avenue have experienced an increase in the total number of crashes. Since most crashes occurred at the two roundabouts are property damage only crashes, removing the two sites from the analysis increases the reduction in total crashes (for all sites) from.% to.% but changes slightly for the injury and fatal crashes, from % to.%. Further review is underway to determine what may have caused the increase in crashes at these two locations. Even for the roundabouts with an increase in injury crashes, the average annual increase was 0. with the largest annual increase being 0.. TRB 0 Annual Meeting

12 Qin, Bill, Chitturi, and Noyce 0 TABLE EB Analysis for Total Crashes Location Intersection Type STH /Gaynor St/th St UrbST CTH F/S. Ninth St. UrbST CTH F/Suburban Dr. UrbST STH / and STH UrbSTALL STH / Allouez Ave RurST STH /STH Broadway UrbSG STH /CTH KK RurST Lake Park/Plank Rd (CTH LP/CTH P) UrbST CTH N / Emons Road RurST STH / RurST STH / I-, Interchange Ramps (West) RurSG STH / I-, Interchange Ramps (East) RurSG STH /Vanguard, Wal-Mart entrance RurSG Breezewood ln/tullar Rd UrbYD US ramps and CTH O (West) UrbST US ramps and CTH O (East) UrbST STH /CTH S RurST Canal St/th Ave UrbSTALL STH /CTH K UrbST Elkhorn Rd (Bus )/Bluff Rd/Clay St UrbYD STH /STH, th St, Springdale, CTH ID UrbSG Thompson and Commercial (North) UrbSTALL Thompson and STH 0 (South) UrbSTALL Old STH /Parmenter UrbSTALL Observed Total Crashes-Before (A) Expected Crashes EB-After (B) Observed Total Crashes-After B-A (Number) B-A (% Reduction=00(B-A)/B) TRB 0 Annual Meeting

13 Qin, Bill, Chitturi, and Noyce TABLE EB Analysis for Fatal and Injury Crashes Location STH /Gaynor St/th St UrbST CTH F/S. Ninth St. UrbST CTH F/Suburban Dr. UrbST STH / and STH UrbSTALL STH / Allouez Ave RurST STH /STH Broadway UrbSG STH /CTH KK (high speed) RurST Lake Park/Plank Rd (CTH LP/CTH P) UrbST CTH N / Emons Road RurST STH / (high speed) RurST STH / I-, Interchange Ramps (West) RurSG STH / I-, Interchange Ramps (East) RurSG STH /Vanguard, Wal-Mart entrance RurSG Breezewood ln/tullar Rd UrbYD US ramps and CTH O (West) UrbST US ramps and CTH O (East) UrbST STH /CTH S RurST Canal St/th Ave UrbSTALL STH /CTH K UrbST Elkhorn Rd (Bus )/Bluff Rd/Clay St UrbYD STH /STH, th St, Springdale, CTH ID UrbSG Thompson and Commercial (North) UrbSTALL Thompson and STH 0 (South) UrbSTALL Old STH /Parmenter UrbSTALL Intersection Type Observed Total Crashes-Before (A) Expected Crashes EB-After (B) Observed Total Crashes-After B-A (Number) B-A (% Reduction=00(B-A)/B) TRB 0 Annual Meeting

14 Qin, Bill, Chitturi, and Noyce In spite of the varying safety performance across individual roundabouts being evaluated, the level of improvement can be associated with the prevailing geometric characteristics and traffic conditions prior to roundabout construction. The following analysis was focused on three aspects: speed, number of lanes, and traffic control strategies. Speed and Roundabout Safety There are perceived concerns with constructing roundabouts on roadways or corridors with highspeed approaches posted at mph or greater. Caution should be taken to control the amount of deflection on a roundabout entry in order to avoid abrupt speed change which may lead to singlevehicle crashes. When the speed differentiation between the roadway and roundabout is greater than mph, additional geometric changes such as compound curve or cross-sectional features may be considered to reduce the approaching vehicle s speed (). Table shows that among the roundabout locations in Wisconsin, have at least one approach with the posted speed limit of mph or higher. The roundabouts with high-speed approaches experienced a crash reduction of percent while the roundabouts with low-speed approaches experienced a 0 percent crash increase. Seven out of high-speed roundabouts have decreased crashes compared to out of of the roundabouts with low-speed approaches. If only fatal and injury crashes are considered, high-speed and low-speed roundabouts are similar: a percent reduction in injury crashes at low-speed roundabouts and a percent reduction at high-speed roundabouts. However, the number of roundabouts in which injury crashes decreased is different: out of high-speed roundabouts decrease in injury crashes compared to out of low-speed roundabouts.. TABLE Speed and Roundabout Safety Total Crashes KABC Crashes Single-lane versus Multi-lane Number of RABs Low-speed High-speed RABs with Increased Crashes RABs with Decreased Crashes Total Expected Crashes Total Observed Crashes 0 % of Changes.% -.% RABs with Increased Crashes RABs with Decreased Crashes Total Expected Crashes 0 Total Observed Crashes % of Changes -.% -.% As shown in Table, five of the single-lane roundabout locations experienced a decrease in all crashes. Six of the multi-lane roundabout locations experienced a decrease in all crashes. While both single and multi-lane roundabouts experienced some crash increase at several sites, total TRB 0 Annual Meeting

15 Qin, Bill, Chitturi, and Noyce 0 0 crashes were different. Multi-lane roundabouts had a percent increase in total crashes; conversely, single-lane roundabouts had a nearly percent reduction in all crashes. The opposite was observed when examining fatal and injury crashes. Only one out of twelve multilane roundabouts experienced an increase in injury crashes compared with six out of twelve of the single-lane roundabouts. Considering injury crashes, multi-lane roundabouts had an overall reduction of percent while single-lane roundabouts showed an percent reduction. TABLE Roundabout Safety Performance by Number of Lanes Total Crashes KABC Crashes Traffic Control Strategies Number of RABs Single-lane Multi-lane RABs with Increased Crashes RABs with Decreased Crashes Total Expected Crashes 0 Total Observed Crashes % of Changes -.%.% RABs with Increased Crashes RABs with Decreased Crashes Total Expected Crashes Total Observed Crashes % of Changes -.0% -.% The NCHRP Report study reported reductions of approximately percent for all crashes and percent for injury crashes when an intersection was converted to a roundabout from a signal or stop control (). The safety benefits however vary considerably among traffic control alternatives, including Yield, two-way stop controlled (TWSC), all-way stop controlled (AWSC), and signal control. The highest safety effectiveness was recorded when an intersection was converted from TWSC as shown in Table. Mixed results were found for other control types. The safety performance is insignificant when converting an AWSC to a roundabout due to its low volume conditions. The conversion from a signalized intersection to a roundabout requires more considerations such as left-turning volume, left-turn storage space, and the space between intersections because the safety benefits are conditional to these unique situations. The conversion of signalized intersections gained marginal safety improvements while more crashes occurred at roundabout locations converted from No Control/Yield and AWSC. From the injury crash perspective, benefits were scored for traffic control of all types. Though injury crashes increased at a few locations with previous control types as TWSC and AWSC, the overall crash reduction is positive. TRB 0 Annual Meeting

16 Qin, Bill, Chitturi, and Noyce TABLE Roundabout Safety Performance by Control Type Before Conversion Total Crashes KABC Crashes CONCLUSIONS Number of RABs No Control/Yield TWSC AWSC Signalized RABs with Increased Crashes RABs with Decreased Crashes 0 Total Expected Crashes Total Observed Crashes 0 0 % of Changes.% -.%.% -.% RABs with Increased Crashes 0 0 RABs with Decreased Crashes Total Expected Crashes Total Observed Crashes 0 % of Changes % -.0% -.0% -.% While roundabouts are still fairly new in the U.S. and Wisconsin, their safety benefits have yielded varied results. For this study, researchers analyzed Wisconsin roundabouts that were built in 00 or before. Three years of before and after crash data were gathered as well as geometric and traffic volume data. An EB observational before and after crash analysis was used to examine the safety benefits for total crashes and injury (K, A, B, C) crashes. The EB analysis was performed using SPFs from both the HSM and SA. In line with other studies, both crash frequency and severity decreased after roundabout installation. Total crashes decreased by percent and fatal and injury crashes reduced by percent. None of the locations had a fatal crash in the after period. Mixed results for total crash frequency were observed. Thirteen locations had an average decrease of. crashes per year and locations had an average increase of. crashes per year. Seventeen locations had a decrease in fatal and injury crashes. The average decrease in fatal and injury crashes was 0. crashes per year. When looking at predictor variables using the Wisconsin roundabout data, the speed limit of the approaches does not appear to contribute to safety issues. While multi-lane roundabouts had a larger decrease in fatal and injury crashes, single lane roundabouts saw a larger decrease in total crashes. TWSC conversions had the highest safety benefit as compared to AWSC and signalized. Given the mixed results, further evaluation and safety inspection including detailed police crash report review and field visit is currently underway in order to identify specific causes for roundabout crashes. ACKNOWLEDGEMENT The authors would like to thank Rebecca Szymkowski, Jerry Zogg, and Patrick Fleming at the Wisconsin Department of Transportation for providing constructive comments and support throughout the study. This work was sponsored by the Wisconsin Department of Transportation. TRB 0 Annual Meeting

17 Qin, Bill, Chitturi, and Noyce REFERENCES. Nine proven crash countermeasures. FHWA, Safety Compass, Fall 00, Volume, Issue.. Maycock, G. and R.D. Hall. Accident at -arm Roundabouts. Report LR 0. Transport and Road Research Laboratory, Crowthorne, Berkshire, United Kingdom,.. NCHRP Report : Roundabouts in the United States. Transportation Research Board. Accessed March, 00.. NCHRP Report : Roundabouts: An Information Guide (second edition). Transportation Research Board. Accessed June, 0.. Persaud, B. N., R. A. Retting, P. E. Garder, and D. Lord. Observational Before-After Study of the Safety Effect of U.S. Roundabout Conversions Using the Empirical Bayes Method. In Transportation Research Record: Journal of the Transportation Research Board, No.. Transportation Research Board of the National Academies, Washington, D.C., 00, pp... Daniels, S., T. Brijs, E. Nuyts, and G. Wets. Explaining variation in safety performance of roundabouts. Accident Analysis and Prevention, Vol. (00), pp Hels, T., and I. Orozova-bekkevold. The effect of roundabout design features on cyclist accident rate. Accident Analysis and Prevention (00), pp Turner, S.A., A. Roozenburg, and T. Francis. Predicting Accident Rates for Cyclists and Pedestrians. Land Transport New Zealand, Robinson, B.W., L. Rodegerdts, W. Scarborough, and W. Kittelson. Roundabouts: an informational guide. U.S. Department of Transportation, Federal Highway Administration, Report FWHA-RD-00-0, Washington, Retting, R. A., B. N. Persaud, and P. E. Garder. Crash and injury reduction following installation of roundabouts in the United States. American Journal of Public Health. (00) (), pp... Highway Safety Manual. American Association of State Highway and Transportation Officials (AASHTO), 00.. Brabander, B. D., E. Nuyts, and L. Vereeck. Road safety effects of roundabouts in Flanders, Journal of Safety Research. Vol. (00), pp... Ogden, K.W., Safer Roads: A Guide to Road Safety Engineering. Ashgate Publishing Limited, Hants ().. Elvik, R. Effects on road safety of converting intersections to roundabouts. Review of evidence from Non-U.S. studies. In Transportation Research Record: Journal of the Transportation Research Board, No., Transportation Research Board of the National Academies, Washington, D.C., 00, pp Mandavilli, S., A. T. McCartt, and R. A. Retting. Crash Patterns and Potential Engineering Countermeasures at Maryland Roundabouts. In Traffic Injury Prevention. Vol. 0, No., Taylor & Francis, Inc, Philadelphia, 00, pp Qin, X., G. Khan, A. Bill, and D. A. Noyce. Comprehensive Safety Evaluation of Roundabouts in Wisconsin. Journal of Transportation Safety & Security, Vol., 0, pp... Bill, A., X. Qin, M. Chitturi, and D. A. Noyce. Comprehensive Evaluation of Wisconsin Roundabouts Volume : Safety Findings, Final Report, University of Wisconsin-Madison, 0. TRB 0 Annual Meeting

18 Qin, Bill, Chitturi, and Noyce 0. Ashmead, D., D. Guth, R. Wall, R. Long, and P. Ponchillia. Street Crossing by Sighted and Blind Pedestrians at a Modern Roundabout. Journal of Transportation Engineering, (), 00, pp... Wall, R., R. Long, D. Guth, D. Ashmead, and P. Ponchillia. Roundabouts: Problems of and Strategies for Access. International Congress Series, September 00,, pp Harkey, D., and D. Carter. Observational Analysis of Pedestrian, Bicyclist, and Motorist Behaviors at Roundabouts in the United States. n Transportation Research Record: Journal of the Transportation Research Board, No., Transportation Research Board of the National Academies, Washington, D.C., 00, pp... Hauer, E. Observational before-after Studies in Road Safety: Estimating the Effect of Highway and Traffic Engineering Measures on Road Safety. Amsterdam, Boston: Pergamon, 00.. WisTransPortal. University of Wisconsin-Madison. Accessed on June 0, 0.. Wisconsin Highway Traffic Volume Data. Wisconsin Department of Transportation. Accessed on May, 0. TRB 0 Annual Meeting