Safety Evaluation of Roundabouts in Georgia

Abstract

U.S. roundabout growth has been significant in recent years and many published studies have documented significant safety benefits of roundabouts. However, the safety benefits for a roundabout may vary from region to region depending on many local factors. Therefore, transportation agencies can make more informed implementation decisions with local safety evaluations rather than published national findings. However, roundabouts are relatively new in the United States and most departments of transportation, including Georgia, are often hindered by the data availability requirements of the state-of-the-art empirical Bayes analysis evaluation procedure. This current study provides a safety evaluation of 23 Georgia roundabouts. It adopts a time-dependent form of the Highway Safety Manual predictive (empirical Bayes) method to estimate potential crash reductions across all crashes and all injury/fatal crashes. The method extends the empirical Bayes procedure towards a full Bayesian analysis. The findings indicate a 37–48% reduction in average crash frequency for all crashes and a 51–60% reduction in average crash frequency for injury/fatal crashes at four-leg roundabouts that were converted from stop-controlled and conventional intersections. In addition, when analyzed as a group, three-leg and four-leg roundabouts converted from stop-controlled and conventional intersections collectively experienced 56% reduction in average crash frequency for all crashes and 69% reduction in injury/fatal crashes. The study did not consider five-leg roundabouts because of small sample size and concerns about the form of the safety performance function. The adopted methodology offers departments of transportation with data availability challenges an alternative evaluation framework that retains the positive attributes of empirical Bayes analysis.