CONVERSION OF A SEMI-TWO LANE ROUNDABOUT INTO TURBO ROUNDABOUT: A SAFETY ANALYSIS

Roundabouts have become a common type of intersection in different countries because they contribute to improve traffic safety and to reduce environmental impacts. Moreover, among all types of intersections and with respect to conventional roundabouts, turbo-roundabouts have been proven to offer higher safety levels in particular traffic conditions by reducing the number of potential conflicting points, without compromising efficiency. A turbo-roundabout is a specific kind of spiraling roundabout characterized by many advantages primarily related to the lower speed of passing vehicles and to the bounding of traffic flows by means of lane dividers. The center island is not traversable or partially traversable and no lane-changing can occur on circulatory carriageway because drivers have to choose direction before entering the intersection. In the light of the abovementioned facts, this paper focuses in the analysis of safety performance after a geometric and functional conversion of an existing semi-two lane roundabout into a turbo-roundabout. Micro-simulation scenarios were set using experimental data on traffic flows, accepted gaps, approaching delays and queue length measured on the case-study roundabout placed in the city of Cosenza (Italy). The parameters of safety performance were obtained by using the micro-simulation tool VISSIM and then analyzed by the software SSAM. Results highlighted that the conversion determines benefits for both safety and operational conditions improvement. Furthermore, the comparison carried out in this study can be helpful for both practitioners and administrators in selecting the most adequate geometric design of a roundabout for the specific needs in terms of safety conditions.