Capacity Evaluation of Roundabout Junctions In Addis Ababa
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Date
2007-04
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Addis Ababa University
Abstract
This thesis addresses the most important element of operational performance of
roundabout traffic intersections in Addis Ababa: 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 by aaSIDRA software. 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 10 roundabouts. These 10 roundabouts represent different sizes of
inscribed circle diameters of roundabouts, which are directly related to their approach
leg numbers (3 leg, 4 legs, 5 legs and 6 legs).
aaSIDRA software capacity analysis results indicated that out of 10 roundabouts 6 of
them have greater than 0.85 degree of saturation. This 0.85 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 10 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
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Keywords
Roundabout ;Junctions