Optimal Traffic Signal Timing Allocation for Urban Congested Roundabouts Case Study at Gerji Emperial, Bole Mikael and Saris Abo Roundabouts

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Addis Ababa University


Traffic congestion is a critical problem in Addis Ababa Ethiopia due to the fast growing economy, car ownership and population in the country. The road improvement practices also contribute to the reduction in roadway capacity which in turn to aggravates traffic congestion and delay on the study areas. Roundabouts are areas, where traffic congestion is severe. The potential cause of traffic delay in these locations includes poor parking spaces and practice, compromised geometric design, absence of traffic signs and signals and pavement markings, absence of traffic police personnel and existence of aggressive driver’s behavior. This research evaluates roundabout sections performance. Study area roundabout are Gerji Emperial, Bole Mikael and Saris Abo. Those study areas were prone to traffic congestion due to excessive demand and poor roadway capacity. The study utilized spot speed study, traffic and pedestrian volume study and roundabout geometry study. Signal timing design and approaching saturation flow rate were computed by using HCM 2000. Excel spreadsheet was used to analyze the collected data. Three alternative mitigation measures were evaluated by using VISSIM and VISTRO simulation tools. Those alternatives are, Alternative 1- roundabout as it is, alternative 2 - changing roundabout into signalized intersection and alternative 3 - roundabout signalization. Roundabouts have in excess of control delay and higher travel time relatively than those two alternative solutions. Alternative 2 and 3 reduce control delay by 34% and 47% from the previous under base and 52% and 69% under forecasted condition for peak hour traffic flow respectively. Thus the proposed engineering solutions enhance and improve selected roundabouts sections performance. Therefore these study findings states that study area roundabouts should be supported by traffic signal timing (roundabout signalization) or demolish roundabouts and changed to signalized intersection. Keywords: traffic congestion, dead lock, signal timing, control delay, queue length.



traffic congestion; dead lock; signal timing; control delay; queue length