Capacity Estimation For Roundabouts Using Gap Acceptance Method: A Case Study On Addis Ababa

Abstract

Capacity of roundabout depends on several factors which include the high traffic volume of vehicle, geometry of roundabout, high pedestrian volume. The main emphasis of this study was focused on the most important element of operational performance of roundabout traffic intersections in Addis Ababa: capacity analysis. The relation between a roundabout performances measure and capacity is often expressed in terms of degree of saturation (Demand volume – Capacity ratio). Capacity estimation is done based on empirical gap – acceptance method that is accepted by aaSIDRA software. The essential geometric data for the analysis (average entry width, circulatory road width, number of entry and circulatory lanes, and island diameter), traffic movement data with vehicle characteristics and pedestrians volume were gathered from 6 roundabouts. These 6 roundabouts represent for different sizes of inscribed circle diameters of roundabouts, which are directly connected to their approach leg numbers (3 leg, 4 legs, 5 legs and 6 legs). aaSIDRA software capacity estimation results indicated that out of 6 roundabouts 4 of them have greater than 0.85 degree of saturation. This 0.85 value is recommended by evaluation process of some model countries such as Australia, Germany, United Kingdom and U.S.A. whose roundabouts were planned to operate at no more than 85 percent of their estimated capacity. Approach entry capacity has been analyzed for all 6 roundabouts at their legs and with curve – fitting techniques. Effective capacity versus geometric parameter relationship have been developed in order to find out the causes of their over saturation (v/c > 0.85) and the results indicate that number of entry lanes, number of circulatory lanes, high traffic volumes are the major causes of over saturation. Moreover, the chart is developed using the parameters number of entry lanes, number of circulatory lanes and opposing circulatory flows, which can assist in designing of roundabouts and forecasting their capacity.