Two-Geometry Roundabouts: Estimation of Capacity

The authors have already developed and studied the Two-Geometry Roundabouts in terms of design principles. This paper in fact is the natural extension of another article named - Two-Geometry Roundabouts: Design Principles - in which the authors discussed the main concepts in the Two-Geometry Roundabouts and the details that determine their success. However, when talking about roundabouts in general, it is also of fundamental importance to talk about the evaluation of their capacity. The Highway Capacity Manual (HCM) provides a rigorous method for estimating the capacity (and assessing the Level of Service) of conventional roundabouts. However, the Two-Geometry Roundabouts are unconventional roundabouts and therefore this paper aims to answer the question: “which model should be used to calculate the capacity of the Two-Geometry Roundabout?”. To do this, the Italian legislation will first be considered and a comparison will be made between it and the HCM methodology. After that, the 2 methods most used to date for calculating the capacity of the Two-Geometry Roundabouts (SETRA method and Brilon-Wu method) will be illustrated. Finally, before drawing conclusions, possible future developments will also be introduced in the context of this research, starting from some simulations made with the Aimsun software on a Two-Geometry Roundabout in Italy.