How to derive the analytical capacity model for not-conventional urban roundabouts

The inapplicability of current methods for analyzing operations of urban not-conventional roundabouts (i.e. not referable to standard schemes) hampers seriously the assessment of their operational performances. Moreover, local constraints and the road network structure have produced lots of geometric layouts that make hard to propose an only framework to explain performances of not-conventional roundabouts. Also for these intersections it is hoped to have scientifically based methods to analyze operations with a proper reliability. Starting from these considerations, the paper shows the conceptual path followed to analyze operations at not-conventional roundabouts along an arterial of Palermo City. The examined intersections have suggested a theoretic-experimental approach that balances the need both to match field observations and to have a general criterion to determine performances. A case study application allows to explain how to derive the analytical capacity model from field data. Moreover the comparison of results to those derived by models for conventional schemes inform us about the effect on capacity aroused by a more realistic operational pattern such as that observed at multilane-large-diameter roundabouts.