Comparison of Safety Performance Models for Urban Roundabouts in Italy and Other Countries

In Italy, almost half of all road crashes occur at intersections, primarily in urban areas. In recent years the widespread conversion of at-grade intersections to roundabouts has brought many safety advantages. To assess the safety benefits of roundabouts, transportation professionals need the powerful statistical tool known as the safety performance function (SPF). To date there has been no reported application of this tool to assess the relative safety performance of Italian urban roundabouts. This paper fills that void by using data sets from two municipalities in northern Italy. SPFs were estimated for each city with the negative binomial error distribution and then recalibrated for application in the other city by using the procedure described in the Highway Safety Manual. Model reliability and between-city transferability were evaluated with the cumulative residuals method. To assess how the Italian roundabout SPFs might be used to learn lessons from differences in crash experience for similar intersections elsewhere, a comparison with models from other countries is also provided. This comparison reveals that Italian roundabouts tend to be less safe. Potential reasons for this finding are explored.