Two Roundabouts are Better than One (Comparison of roundabout designs for a complex rural interchange in Tuscany, Italy)

Abstract This paper summarizes the calculations and considerations for a comparative assessment between three alternative proposals for redesign of the interchange between the Pisa North toll plaza of Motorways A11 and A12 and the Via Aurelia (highway SS1) located in the Migliarino region of Tuscany, Italy. The complex interchange also includes access to a local minor road, (Via Traversagnola) and is topographically constrained by the proximity of an important drainage channel which runs alongside the Via Traversagnola. Three designs are evaluated: first, a large elliptical layout, second, a very large circular layout, and third, a layout with twin large roundabouts and slip bypass ramps. These three designs were actually proposed by two different Italian Highway Agencies. Comparisons are made through the respective proper analysis model (i.e., British model for the elliptical traffic circle and French SETRA model for the twin roundabout solution and the very large circular layout) evaluating capacity and performance requirements. Results are summarized and they highlighted that the twin roundabout design is assessed as the preferred solution for traffic operations, as well as safety, maintenance, visual intrusion, and lighting requirements. Finally, from a general point of view, the paper is an applied example that supports the basic idea of distributing maneuvers and conflicts on two facilities is often better than trying to cope with them in a single, even very large facility, mixing flows altogether at the same time and place.