Now days, traffic congestion is becoming a serious issue in developing countries in general andrnin Addis Ababa in particular. Its major impact on the day-to-day activities of road users is notrnlimited to delay in travel time but also it affects the consumption of fuel and the surroundingrnenvironment in terms of greenhouse gas emissions. rnIn Addis Ababa, the issue is becoming a great concern to its residents, transport managers andrnplanners. In light of the interest in testing the performance and optimization of SignalizedrnIntersection timing plan on major route corridor of Addis Ababa, this study was designed andrnconducted on Ras Mekonnen Street between the Addis Ababa School of Commerce andrnEstifanos, as it is the most signalized route corridor in the city and serves the cityâ€™s major trafficrnpopulation. rnBased on Highway Capacity Manual (HCM), the current performances of signal operation on thernroute corridor were assessed along with the cost of congestion results from Travel Time Lost,rnExtra Fuel Consumption and Environmental Pollution costs. In addition, using Synchro 9rnSimulation Software, the Intersection signal timing plan was optimized with respect to networkrncycle length, offset and splits. rnThe result of performance evaluation indicated that all the Signalized Intersections under thisrnstudy serve the traffic demand with poor level of service (LOS F) resulting in costs to thernCountry amounted to ETB:1,191,442.76 per hour due to extra fuel consumption and travel timernloss and also affected by pollutions with emission rate of 31,668.64 g/h of NOx; 206,796.22 g/hrnof HC; 2,874,467.45 g/h of CO; 3,736,807.68 g/h of PM2.5 and 134,375,604.21 g/h of CO2rnattributed by the on roads traffic on the study segment. rnThe simulation analysis for optimization shows that service quality at intersections can bernimproved by changing the current operating cycle length of 188sec to 270sec for EstifanosrnIntersection and 230sec for all other Intersections in the network, together with phase sequencesrnand splits. As a result, more than 550 vehicles will be relieved from stopping at intersections andrnalso extra 654 litters of fuel consumed by the vehicles per hour will also be saved owing tornimprovement of signal timing.