Safety diagnostic method in urban circular intersections

The available knowledge of traffic safety at roundabouts is not a complete reference for geometric design; where there are organizational schemes for junctions characterized by a geometric design, quite different driving behaviours are required from those that are observable at roundabouts. This is very frequent in urban areas, where the existing constraints, particularly physical ones, often render it necessary to make “compromise choices” relating to one or more geometric features of the roundabout (central island shape, circulatory roadway width, symmetry and number of the arms, entry width, etc). The objective of this study is to explain the accident phenomenology of this kind of road scheme, with reference to the most frequent accidents for the junctions examined in different traffic and risk exposure conditions. The study also intends to verify the effectiveness of risk analysis by means of an infrastructural scenarios method, devised in previous research by the author. In this paper, in order to deduce the contribution of the accident-causing factors to each type of accident and to evaluate the relative contribution in determining the level of risk for the different scenarios, the risk model has been checked considering specific accident classes. The results confirm the applicability of the risk evaluation method to urban road schemes, characterized by peculiar geometric features such as the impossibility of applying the safety methods proposed in the literature regarding traditional roundabouts; moreover, in the terms of greatest interest to road designers, they provide useful elements for evaluating the effects of deviations from design rules on urban road safety.