The Effects of Traffic Mixes with 3-Wheelers on Performance of Unsignalized Intersections in Harar City

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In heterogeneous traffic conditions, the performances of an intersection are described by vehicle and driver characteristics which are different from traffic conditions in homogeneous conditions. The absence of lane discipline results in vehicular movement that is influenced by the presence of vehicles in the front as well as on the sides. This led to a complex traffic behavior and it cannot be analyzed by using conventional microscopic and macroscopic traffic variables. In this study the relationship between different vehicle compositions and the performance measurement parameters for two roundabouts and one unsignalized T-intersection in Harar city are assessed by application of gap acceptance procedures. All data which describes actual condition of the intersections were collected to calibrate the existing condition into gap acceptance procedures of highway capacity manual. Critical gaps follow up times and passenger car equivalents representing the existing traffic conditions of the study area are estimated. The analysis is made in 15minute interval over a range of 24 consecutive time series to indicate the trends of change of the performance of intersections in relation to the change of vehicle compositions and flow characteristics. Highway capacity manual 2000 was used for measurement of performances and multiple regressions was the analysis method applied to indicate the effects of vehicle classes in the study The results are interpreted by five performance measurement parameters. They are approach capacity, degree of saturation, queue length, control delay and LOS. The model which indicates the relationship between these parameters and the vehicle compositions are formulated in the form of multiple linear regression equations with R 2 greater than 0.85 for most approaches of the selected intersections. Cross validation was made on the models to estimate how accurately a predictive model will perform in practice. Peak hour performances are compared with the presence of bajaj vehicles and with bajajs replaced by minibus taxis. Finally, the result indicates that some of the approaches of the intersections selected are providing service closer or above the capacity during peak hour of traffic flows. Besides, the presences of Bajajs were significantly affecting the performances of the intersection. A replacement of bajaj vehicles by 12 seat minibus taxis indicates a very good improvement of performances of the intersections in Harar city. For Arategna roundabout the average delay will be reduced by 7 minutes per vehicle if bajajs are replaced by minibus taxis during peak hour. The queue lengths will be reduced by 85 vehicles and the capacity will be increased by 396veh/hr for peak periods. Sillassie roundabout will be improved by decrease of average delay by 95 sec/veh, decrease of queue length by 34 vehicles and increase of capacity by 531veh/hr during peak hour if bajajs are replaced by minibus taxis. Thus, the effect of traffic mix with three wheelers is very significant on performances of unsignalized intersections.



Unsignalized, Bajajs (3-wheelers), Roundabout, Composition, Delay, Performance, Intersection, Traffic Flow, Capacity