In Transportation, Signalized Intersections are an important nodes of traffic movement and thisrnnode are designed and their performance evaluated based on the adopted standards. In thisrnrespect, most of the parameters that are used for design and capacity evaluation of SignalizedrnIntersection are taken from Highway Capacity Manual (HCM) which was developed byrnTransportation Research Board in Washington, D.C. Accordingly, the default values will notrnrepresent the actual condition of the traffic for a specific study area. To this effect, calibration ofrnthe values should need to be the major task for researchers and traffic agencies. Lost time is onernof the key parameters used to estimate an optimum cycle time. It is a very sensitive parameter andrnsmall variation of lost time may cause large changes of cycle time. rnrnDifferent studies show a great deal of variations in start up lost time and saturation flow. Thisrnindicates a lack of stability and this is acknowledged in the HCM. Due to these instabilities, HCM rnrecommends that local data collection be performed to produce more accurate estimates of localrnstart-up lost time and saturation flow.rnrnIn this Study, experimental research is carried out for estimating the start-up lost time and therncontributing factors affecting its value. For this purpose, observations were carried out on fourrnselected signalized intersections in Addis Ababa City. Data was collected by a video recording ofrntraffic flow at four selected signalized intersections of Addis Ababa city. Recorded data werernanalyzed for start-up lost time by extracting the information of time headway of the vehicles. Inrnthis research a more accurate way of estimating Start Up Lost Time is proposed by developingrnapplicable and replicable models. rnrnThe analysis showed that the overall estimated mean of start-up lost time is 2.513 Sec for allrnobserved sites with a different mix of traffic and Geometric condition as well as from the analyzedrnintersections the ideal intersections were selected as per HCM recommendation and it shows thatrnthe mean start up lost time for an ideal intersections is 2.383 Sec and t test showed that the meanrnstart up lost time is significantly different with the HCM recommendation of 2sec per phase. StartrnUp Lost Time data for eleven vehicular compositions were fitted to a probability distribution.rnLognormal, Weibull and Johnson Transformed were found to be best fitted probability distributionrnfor the Start Up Lost Time. Regression Analysis was conducted by Multiple Linear Regressionrntechnique to predict Start Up Lost Time. From regression analysis Mix of Traffic (VehicularrnComposition), Approach Grade, Lane Width, Approach Width, Green Time, Type of Movement,rnLane Position for Maneuvering and Saturation Headway were the factors that affect start up lostrntime and found statistically significant to predict Start Up Lost Time.