Evaluation of Capacity and Performance of Selected Roundabouts: A Case Study in Addis Ababa City

Abstract

The growth and development of metropolitan cities imply a pragmatic increase in all dimensions, causing many challenges especially in transportation systems. Among these, the problems of traffic congestions, long time delay and inappropriate level of services are experiencing by the travelling public every day. In Ethiopia, these problems have been a significant issue which has a great impact on human and economy of the country. To investigate and identify these problems, this research has evaluated the capacity and performance of selected roundabouts in Addis Ababa city. The methodology that has been used for the study was quantitative and qualitative research design method. The collected data for the survey was road geometry data, peak hour traffic volume including pedestrian, while the secondary data was taken from different sources. Traffic volumes data were collected at peak hour within 15minutes intervals starting from 8:00 am - 9:00 am and 5:30 pm - 6:30 pm for five consecutive working days. From these, the three successive working days except Monday and Friday traffic data used in the analysis. Vehicles were classified during traffic collection and converted to the same vehicles category using passengers' care unit (PCU) of each vehicle class. The analysis has been conducted using both manual calculation based on HCM 2010 and SIDRAINTERSECTION software version 5.1. The results obtained from both methods were compared based on the performance measures such as delay, LOS, a degree of saturation and queue length. The results indicated that the analysis using software had shown over-saturated traffic flows, long time delay and the high degree of saturation than using manual calculation method. The reason was due to the calibrated of environmental factors, vehicles and population growth rate embedded as default in the SIDRA INTERSECTION Software. From the analyzed results the existing capacity and performance of Abune Petros and German(Mekanisa) roundabouts were over saturated, long time delay, long queue length and operates beyond their capacity(LOS F.) Hence, it is concluded that factors affecting capacity and performance of roundabouts are high traffic flow, unbalanced heavy vehicles proportion, island diameter, and number of lanes, lane width and a number of circulatory lanes. To improve the capacity and performance level; Abune Petros roundabout would be upgraded by increasing island diameter from 27.8m to 80m, lane width to 6m and number of circulating lane from 2 to 3, whereas German roundabout was recommended to be changed to signalized intersection