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Effects on Road Safety of Converting Intersections to Roundabouts: Review of Evidence from Non-U.S. Studies

A meta-analysis of studies reported outside the United States was performed to evaluate the effects on road safety of converting intersections to roundabouts. Twenty-eight studies that provided 113 estimates of effect were evaluated. State-of-the-art techniques of meta-analysis were applied to synthesize evidence from these evaluation studies. A meta-regression analysis was performed, and the possible presence of publication bias was tested and adjusted using the trim-and-fill method. The results show that roundabouts are associated with a 30% to 50% reduction in the number of injury accidents. Fatal accidents are reduced by 50% to 70%. Effects on property damage accidents are highly uncertain, but in three-leg intersections, an increase often will occur. Evidence from the evaluation studies, although highly uncertain, suggests that the effect of roundabouts on injury accidents is greater in four-leg intersections than in three-leg intersections, and it is greater in intersections previously controlled by yield signs than in intersections previously controlled by traffic signals. Few studies have evaluated in detail the effects on safety of design parameters for roundabouts. Findings are inconsistent, but the majority of studies find that small roundabouts (a small diameter of the central traffic island) are safer than large roundabouts (a large diameter of the central traffic island).