Transportation Safety

Summary:

The objective of this research was to evaluate driver behavior, safety benefits, operational and environmental benefits of roundabouts in Michigan. To that end, a list of all 180 known roundabouts was compiled from various sources. For comparison purposes, a group of reference intersections was also compiled that had similar traffic volume, roadway characteristics, and traffic control prior to roundabout construction. Field data collection was carried out at 18 sites that included collection of driver speed selection behavior, gap acceptance and rejection behavior, and yielding behavior towards traffic in circle and also towards pedestrians at roundabout crossings. Results showed that roundabout geometry significantly affects speed selection behavior of drivers as they approach the yield line. In terms of gap behavior, drivers tended to accept smaller gaps at multilane roundabouts, three-legged roundabouts, roundabouts in rural areas, and roundabouts that were not on interchange compared to their counterparts. Yield rates towards both pedestrians and traffic in circle tended to be the lowest on roundabouts located on interchange. In terms of safety, roundabouts were found to reduce crash severity and reduce the proportion of certain crash types. Safety analysis was carried out at various levels of detail including naïve before-after, empirical Bayes (EB) method, and cross-sectional analysis. However, all the approaches showed that conversion of an intersection to a roundabout significantly increased the number of total crashes but reduced the number of fatal and injury crashes. These trends were generally comparable to prior roundabout study carried out by Michigan Department of Transportation (MDOT) published in 2011. Safety performance functions (SPF) were also developed for roundabouts based on the number of approach legs and number of circulating lanes. Roundabouts were also found to have significant operational and environmental benefits in terms of reduced delay, better level of service (LOS), and fuel savings. Lastly, economic analyses were also conducted to develop benefit-cost curves and estimate benefit/cost ratios for select roundabouts. The research results are intended to guide MDOT and other road agencies in deciding the need to convert an existing intersection to a roundabout.

Publications:

• Final Report
• Spotlight