Abstract:
This independent project addresses the most important element of operational performance of roundabout
traffic intersections in Jimma: capacity analysis. The relation between a roundabout performance measure
and capacity is often expressed in terms of degree of saturation (Demand volume – Capacity ratio).
The capacity analysis is done based on empirical gap – acceptance method that is adopted using
manually. The necessary 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 collected from Meneharia roundabouts. This roundabout
represent different sizes of inscribed circle diameters of roundabouts, which are directly related to their
approach leg numbers four legs.
The capacity analysis result indicated that out of Meneharia roundabout have greater than 0.66 degree of
saturation. This 0.66 value is recommended by analysis procedure of some model countries such as
Australia, Germany, United Kingdom and U.S.A. Whose roundabouts are designed to operate at no more
than 85 percents of their estimated capacity.
Approach entry capacity has been analyzed for all Meneharia roundabouts at their legs and with curve –
fitting techniques. Effective capacity verses geometric parameters relationship have been developed in
order to find out the causes of their over Saturation (v/c ratio greater than 0.85) And the result indicates;
number of entry lanes, number of circulatory lanes, high traffic flow and pedestrian volumes are the major
causes of their over saturation.
Furthermore, 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.
