Roundabout Deflection: Testing and Improving the Italian Standard

Roundabouts have become a popular option to conventional intersection design in Italy, as well as in the rest of the world. They are used in both urban and rural roads for the efficient and safe control of traffic, for improving safety by reducing the approaching and the circulating speed at the intersection (as traffic calming device), for minimizing the maintenance cost (compared with signalized intersection). Achieving appropriate vehicular speeds through the roundabout is the most critical design objective, especially in Europe where they are mainly used for reason of road safety. The parameter used by the Italian Standard to control speeds through the roundabout is the deviation angle (45°) but practitioners, often at the end of the project, realize that the recommendation is not satisfied and they are forced to change the geometry, repeating the entire design process or to ignore the recommendation. The paper, after describing some inconsistencies of the Italian Standard, compares some of the speed control methods provided for other international standards and, varying the main geometric parameters of the intersection, checks the deviation angle for more than 7,000 different possible roundabouts in order to provide an helpful guidance for practitioners during the design process. The study refers to one-lane roundabouts with four arms in rural areas. Some recommendations are provided in order to improve the Italian roundabouts Standard which currently do not take into account the type of area or the type of roads affected by the intersection.