Vehicle-size Selection For Road-based Transit Operating In Mixed Traffic –the Case Of Nairobi Thika Highway

Abstract

Vehicle size and type is one of the principal factors influencing the efficiency of transit service. This study sought to develop an approach to the optimization and selection of transit vehicle sizes for an uncoordinated system operating in mixed traffic. Data was collected along an 11.4km portion of the Nairobi-Thika Highway in Kenya. The 11.4km portion was divided into four segments which are defined by major intersections along the highway. Data collected included classified traffic counts, visual occupancy surveys, speed measurements and the measurement of vehicle-plinth areas. The data collected was analyzed to determine ridership patterns along the highway, occupancy patterns for different vehicle types and Passenger Car Equivalencies for different vehicle types. The relationships between occupancy rates and vehicle size; and between Passenger Car Equivalence values and vehicle size were then established. A model was developed that optimizes transit vehicle size based on occupancy rates achieved by different transit vehicle sizes and the road occupancy of a vehicle. The premise of the model is to maximize passenger occupancy in a vehicle while minimizing the road occupancy of the vehicle. Optimum vehicle sizes were determined for the first two segments as 45 and 56 seats respectively. These would minimize roadway congestion along these segments. Along segments 3 and 4, the flows that would result from the selection of a particular vehicle size for the peak passenger flows were established. Further investigations should be carried out on the effects of different vehicle sizes at bus stops. These include the passenger waiting times, berth occupancy, boarding and alighting times, among others