Analysis of Capacity and Traffic Flow Characteristics of Mixed Traffic on Addis Ababa Ring Road (original) (raw)

Detail understanding and analysis of traffic stream characteristics is necessary for design, analysis, operation and management of roadway facilities. Analysis of traffic flow helps to develop mathematical relationship among primary elements of a traffic stream; flow, speed, and density. Flowdensity-speed relation models are used to analyze traffic flow stability, to determine design capacities for roads, level of service and to calculate travel costs. In Ethiopia, capacity, passenger car unit and speed-flow-density relationship are used for design and operation are derived from international standards especially from United States Highway Capacity Manual (HCM), which is empirically derived from condition in North America, may not represent flow condition in Ethiopia. It is important to calibrate this relationship and developing Local Highway Capacity. Thus aim of this paper is to determine the traffic flow characteristics, capacity and passenger car unit that reflect the local traffic conditions in Addis Ababa. This study was carried out through a field data collection from three different study sites located on Addis Ababa Ring Road. The selected study sections are basic segment in flat terrain. The data were collected by means of a video technique, from both directions during peak morning and afternoon hours. The vehicles are classified as Standard Car, big Car, Minibus, Bus, Truck and two wheelers. For each vehicle, data such as, vehicle class; number of vehicles and travel time were measured from vehicle travelled in study section between two thick white lines marked across the road width to provide reference line to arriving vehicles. The measured travel time was averaged for a one minute time interval to determine the average mean speed. Density was computed from number of vehicle counted in given time interval and vehicle speed. In relation to models developed by different researchers to describe the relationship between traffic flow characteristics were tested using collected data in order to investigate which model fit the data satisfactorily for urban multilane highway. Regression curves are analyzed based on four classical models: Greenshields, Greenberg, Underwood, and Bell-shaped models. From the analysis it can be seen that Greenshields and Underwood models are presented with more consistent and less variation. The present study includes estimation of capacity by Greenshields method using field data and by developing simulation model, for urban multilane highway. Traffic volume and speed were determined for peak hour of study area. The flow density relationship is developed for the study area. Study passenger car unit of different categories of vehicles at different volume level were evaluated using microsimulation model VISSIM. It has been found that PCU's decrease with increase in volume to capacity ratio for heavy vehicles. Finally, capacity obtained by both methods for study sections were compared with value suggested by International standards.