TURBO-ROUNDABOUT GENERAL DESIGN CRITERIA AND FUNCTIONAL PRINCIPLES: CASE STUDIES FROM REAL WORLD

Compared with usual roundabouts the main advantages of turbo-roundabouts are the reduction in the number of potential conflicting points and the lower speed of passing vehicles through the intersection; they both can improve road safety conditions at the junction. Also, the physical delimitation among lanes limits the side-by-side accident risk. 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. In a previous paper authors proposed a theoretical approach to evaluate turbo-roundabouts capacity founded on gap acceptance theory. In particular, to model in a realistic way traffic conditions at turbo-roundabouts, simulations have been developed starting from behavioral parameters (critical gap and follow-up time) obtained by field observations on the few existing turbo roundabouts. The determination of the advantage domain (in terms of capacity) of turbo-roundabouts compared with typical ones and the way to value performance indicators (delays, queue lengths and levels of service) more consistent with real operational conditions of turbo roundabouts were also proposed. The present paper has three main objectives: 1. to discuss general design criteria and functional principles of turbo roundabouts; 2. to give the geometric design principles of the central island and circulating lanes; 3. to present three case studies from real world concerning the conversion of existing roundabouts into turbo roundabouts. Although results of the study cannot be generalized, by methodological point of view they can be useful to the practitioners and to the road administrations in decision making about the conversion of existing intersections.