Abstract:
A competitive, growing economy requires a transportation system that can move people, goods,
and services quickly and effectively. These transportation systems have different components one of
which is junction. Junctions are usually considered as the critical points within the urban road
network and the evaluation of their performance provides valuable understanding and useful
indication about the performance of the network.Roundabout is a junction with a central island
where the conflicting traffic streams are separated in time by the priority rules, i.e. the entry vehicles
should give ways to circulating vehicles. Jimma, as one of growing city in Ethiopia, has traffic
volumes which continues to grow while the available lane-miles remain relatively steady, creating a
need for ways to allow more vehicles in an already congested transportation system. This research
was conducted with the objective of evaluating the capacity and level of service of major roundabouts
in Jimma city.
The capacity analysis was` done based on analytical gap-acceptance method that is adopted
by SIDRA intersection version 5.1 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 three
roundabouts having different size and approach leg. The result and value of this study showed that
from the studied intersection, Oilibia roundabout is serving at capacity (LOS F) and that it is
necessary to plan changing it with signalized intersection or increase its capacity. The other two
intersections, namely Aratu Anbessa Intersection and Honeyland Intersection are found to have a
volume-capacity ratio of 0.395 and 0.285, below 0.85, represented a Level of Service “A”. but their
capacity analysis should be conducted after some time to check its consistence.Moreover, Jimma
town municipality should construct sufficient circulatory number of lanes within the roundabout
which is equal to the number of approach leg to avoid longer delay of vehicles at Oilibia intersection
and conduct regular traffic volume count for all intersections in the city to establish database.
