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.
