Performance analysis of basic turbo-roundabouts in urban context

A turbo roundabout is a new type of canalized multilane intersection in which the physical separation between lanes, both at entries and on the ring, helps to prevent side collisions when crossing the roundabout. If compared with traditional roundabouts, the main benefits of a turbo roundabout are: - lower number of potential conflict points between vehicles; - slower speed along the ring; - lower risk of side-by-side accidents at the ring. In light of these considerations, turbo roundabouts could be an alternative to two-lane roundabouts, especially to guarantee a high safety level, for example in case of quite heavy pedestrian traffic in urban contexts. Different variants of the turbo roundabout are obtained by varying the number of lanes at the entry and exit legs; the principal layouts are: i) egg roundabout; ii) basic turbo roundabout; iii) spiral roundabout; iv) knee roundabout; v) rotor roundabout. This paper presents an estimation of capacity, delays and level of service of basic turbo roundabouts in undersaturation conditions, considering both vehicle flow and pedestrian traffic. The traffic performance model was developed by evaluating the capacity for each entry lane. Owing to the geometric features of the intersection, the total entry capacity is obtained by considering different values of the pedestrian impedance factor and degree of saturation at both the right-turn and left-turn lanes. Finally numerical examples are shown to illustrate our results.