Capacity and Queue Modeling in Unsignalised Roundabouts

Capacity and Queue Modeling in Unsignalised Roundabouts


R Camus, M Dall'Acqua, G Longo, University of Trieste, IT



The paper deals with the problem of roundabout?s performances evaluation, mainly focusing on capacity and Level of Service (LOS) attributes. As known, scientific literature and National Laws present different approaches and methods for the design of circular intersections. These estimation capacity methods developed by various countries give not homogeneous results, mainly under peak traffic conditions, and so a correct evaluation of the facility performances is not easy to obtain.

In this context, the paper focuses on evaluating a possible new approach to represent the input legs and the circulatory area of a roundabout facility. This procedure is based on the Highway Capacity Manual 2000 (HCM 2000) time-space evaluation of the traffic dynamics, referring to particular time interval sets and to a specific analysis of the driver behaviour on the merging, weaving and diverging segment procedures.

In this case the roundabout facility is divided into different basic segments according to HCM criteria. In particular for the roundabouts these basic segments could mainly be: on-ramp junction segment, off-ramp segment and weaving areas. The behaviour of each segment is actually described within the HCM with reference to freeways. In this paper these concepts will be discussed in order to meet roundabouts specific requirements. Given that, the time-space domain of a roundabout facility has to be described with particular attention to facility geometrics and traffic demands, in order to perform correct demand-capacity analysis, queue predictions and Level of Service estimations. In such a way, the proposed approach considers the roundabout as a multi-junction node, made of a near succession of different segment areas; each area is studied in detail in order to give a performance measure of the entire facility on the main (and minor) stream level.

With reference to some existing roundabout facilities charged by critical traffic conditions, first results on the application of this procedure are briefly discussed and compared to the results of the existing approaches developed in various countries.


Association for European Transport