A Practical Method for Assessing Sustainable Traffic Levels of Roundabouts in Urban Areas Using the Macroscopic Fundamental Diagram

In traffic engineering, queue lengths, waiting times, and delay values are denoted as measures of effectiveness (MOEs). In the case of roundabouts, MOEs are generally calculated starting from the gap-acceptance theory in which the driver behaviour needs to be properly modelled. This research presents an unconventional technique, based on Macroscopic Fundamental Diagrams (MFDs), to describe sustainable traffic levels and (MOEs) of roundabouts, in terms of capacity, speed, density and delay. The deduction of MFDs is based on relatively simplified theoretical hypotheses, but the outcomes could be of interest in several practical applications. MFDs are estimated in the cases of single and double-lane roundabouts, modelled by microscopic simulations starting from a real-world case study in Italy. MFDs are obtained for a wider sample size of origin-destination traffic matrices taking into consideration time intervals of 5 min and 1 h. The results of this research demonstrate that the traffic distribution between arms can influence the shape of a roundabout-MFD. However, the proposed procedure allows us to evaluate the performance of a roundabout as a whole, taking into account numerous MOEs including the total capacity, the critical density and the Level of Service (LOS). Therefore, the use of MFDs could represent a complementary procedure for evaluating the performance of roundabouts.