Turbo-Roundabouts: a Model to Evaluate Capacity, Delays, Queues and Level of Service

A turbo roundabout is a particular type of roundabout where entering and circulating lanes are bounded by traffic signs and by non-mountable curbs. The physical separation between lanes, both at entries and on the ring, helps to prevent side collisions crossing the roundabout. The main advantages of turbo roundabouts are: i) reduction in the number of potential conflict points; ii) lower speed of vehicles passing through the intersection; iii) safety conditions at the intersection due to lower risk of side-by-side accidents. These aspects make turbo-roundabouts more appropriate than roundabouts when a higher level of safety has to be guaranteed, particularly in presence of relevant pedestrian and two-wheels traffic volumes. Also, in some cases the capacities of turbo roundabouts are higher than the capacities of conventional roundabouts. This paper presents an estimation of capacity, delays, queues and level of service of turbo roundabouts in undersaturation traffic conditions, considering both vehicular flow and pedestrian stream. It also examines the geometric parameters in several turbo roundabout layouts and presents one case study from real world concerning the conversion of existing large diameter roundabout into two linked turbo roundabouts.