Evaluating Safety Performances of Rural Roundabouts via Conflict Opportunities Technique

Abstract

Roundabouts, when properly designed, can provide substantial safety benefits as compared with traditional intersections. Geometric design plays a key role in inducing drivers to reduce their speed when approaching and guiding through roundabouts. Safety Performance Functions and Traffic Conflicts analyses are the most diffused approaches for identifying parameters actually conditioning safety performances of roundabouts. In this study, an alternative model is proposed, based on estimation of potentially hazardous collisions, i.e. Conflict Opportunities, via probabilistic theory, rather than analyses of video recordings. Calculation of Conflict Opportunities directly takes into account daily changes of traffic flows and differences among free traffic flows periods and situations where both entering and conflicting traffic flows are high. Forty-three rural roundabouts were sampled, and, for each leg, 21 geometrical features, selected from technical literature, were collected. For the most frequent types of vehicle crashes, Conflict Opportunity rates were converted into estimated crash rates by means of calibration functions, where independent variables are the geometric features found to be correlated with crash rates. In this regard, exploratory data analyses were particularly useful. They allow maximizing insight into the data set and uncovering underlying structures difficult to capture. Among the various applied techniques, Discriminant Analyses and Multiple regressions with different hypotheses about distribution of residuals proved to be very proficient in identifying a restricted number of significant geometric variables. The model seems to perform well for the considered types of motor vehicle crashes, expect for circulating exiting crashes, probably affected by too few data in the collected sample.