A study of the traffic operational, design and safety characteristics of single lane roundabouts in the United States

Roundabouts are a new form of at-grade intersection control that have recently in the United States. Roundabouts are similar to traffic circles that were common in the U.S. in the 1940's and 1950's. Roundabouts, however, include design and operational improvements that help them perform better in terms of operational and safety performance.; Three objectives were addressed as part of this study: (1) determine the safety performance of single lane roundabouts in the U.S.; (2) model the operational performance of single lane roundabouts in the U.S. including approach capacity and delay; (3) determine geometric features that influence operational and safety performance of roundabouts.; The safety study revealed that single lane roundabouts are performing better in terms of reducing accident frequency and rate. In addition, injury accidents were reduced in the after period when the study intersections were controlled as single lane roundabouts. These findings are encouraging, as they replicate roundabout performance in many other countries.; Gap acceptance theory was next used to model capacity and delay. Gap acceptance theory requires three components to be modeled in order to model capacity and delay: (1) headways in the circulating stream; (2) mean critical gap; (3) mean follow-up time.; These three components were modeled according to field measurements made at six study sites. A gap acceptance based capacity model was developed using the lognormal density function for the distribution of headways in the circulating stream. In addition, a delay estimation model was developed using conditional expectation and renewal theory. This study found that the lognormal density function best replicates field conditions and not the exponential density function, as many models assume.; Finally, the effect of geometric design on operational and safety performance was addressed through a review of accident reports and field observations made at several roundabouts in the U.S. From this study it was found that high speed rural roundabouts may benefit from the use of successive reverse curves on approaches to roundabouts to reduce speeds prior to entry. In addition, it was found that roundabouts operating at very low volume to capacity levels can result in erratic driver behavior including wrong way maneuvers and high speed entries.