Performance Evaluation of Grade Separation Within the Selected Major Intersection in Addis Ababa City

Urban City Traffic Management introduced various remedies to decongest traffic and to reduce the severity of accidents, such as providing traffic signal controls at the intersecting roads at different levels, or grade separating the traffic movement towards different directions. One type of grade separation is Road Underpass or Flyover. The construction of Road Underpass or Flyover poses some doubts whether it is effective or ineffective on the point of views of the travelers or motorists. It is for this reason that this research sought to evaluate the performance and effectiveness of Road Underpass structure on traffic flow at the major intersection, specifically the Wellosefer intersection in Addis Ababa City. The primary data used the existing traffic volume recorded between 7:00 AM – 6:00 PM from Monday to Friday. While the secondary data obtained from the Consulting Office "before" the improvement of Wellosefer intersection, including the previous study, which served as the control analysis for the research. From these data, the analyses provided results on vehicle travel time and delay for a case "before" and "after" improvement of the intersection. SIDRA Software was used to analyze the traffic conditions of the intersection. Based on the findings of the study, a case "before" Improvement of Wellosefer Unsignalized T-Intersection indicated with an average delay of vehicles of about 535.7 seconds, and a travel time of vehicles of about 574.7 seconds. On the other hand, a case "after" Improvement of Wellosefer intersection, three alternatives considered; such as 1) Signalized T-Intersection, 2) Roundabout, and 3) Road Underpass with Roundabout at the upper level. The results of the first alternative showed the Signalized T-intersection indicated an average delay and travel time of 344.3 seconds and 365.0 seconds, respectively. It means, there could be a reduction of 15.4% in average delay if signalized intersection implemented. The second alternative, a case of Roundabout using the same traffic data at current condition, the results indicated the motorists and commuters would be experiencing an average delay of about 355.8 seconds and average travel time of 390.0 seconds. The third alternative considering Road Underpass with Roundabout at the upper level, the results revealed that there was a significant reduction in average delay of vehicles of about 42% from the Unsignalized T-Intersection. Comparing this result with the Signalized intersection using the same traffic data, it showed that there was a significant decrease in average delay. Therefore, the findings of the study, a combination of Road Underpass and Roundabout at the upper level as in the case of Wellosefer intersection performed well than the other traffic control design alternatives.