Evaluating capacity and efficiency of turbo roundabouts

Recent evolution of roundabout design has been directed towards the so-called turbo-roundabout, with the aim of ensuring better safety conditions than those afforded by traditional schemes. The few turbo-roundabout installations so far implemented have not allowed sufficient research in order to analyze operational conditions and capacity models validated by field observations. In any case the theoretical approach to the functional study of turbo-roundabouts undoubtedly involves more complex aspects compared to typical roundabouts because of the paths of entering streams (constrained to get onto physically separated lanes), the “turbine” configuration of the circulatory carriageway, as well as the right-of-way regime imposed on entering traffic. So the conflicting schemes between entering and circulating flows appear highly modified. And so do, consequently, the capacity models and the criteria for evaluating performance which can be applied. Considering these points and the state-of-the-art knowledge on the subject, this paper proposes a criterion for evaluation of turbo-roundabouts capacity which is based on some previous models for unsignalized intersections (particularly roundabouts and Two-Way-Stop-Controlled intersections) and validly tested by the gap-acceptance theory. Though theoretical, the proposed approach has the following main objectives: i) determination of the advantage domain (in terms of capacity) of turbo-roundabouts compared with typical ones; ii) evaluation of performance indicators (delays, queue lengths and levels of service) more consistent with real operational conditions of turbo-roundabouts; iii) the possibility of establishing a homogeneous comparison between traffic performances at turbo-roundabouts and usual uncontrolled intersections.