Modelling signal design parameters under heterogeneous traffic condition

Abstract

This study is aimed at the determination of PCU values, saturation flows, and quantifies their impacts in traffic stream at selected signalized intersection of Dhaka city. This paper is also focused on the process of research work, particularly on the general methodology, site selection, data collection and analysis. Saturation flow and PCU factors were determined using total width concept instead of lane-based concept. Data were obtained by video recording and analyzed by regression analysis. The paper also discusses the effect of geometric and local environmental factors, traffic conditions and vehicle-related factors on saturation flow and PCU values at signalized intersections. Proportion of motorized and non-motorized vehicles, approach width, turning radius and turning proportions are the principal factors for which saturation flow and PCU factors are varied. The results and analysis show that PCU values are not unique for a particular vehicle type, rather it is dependent on traffic movement, geometric features, vehicle performance etc. It is observed from the analysis that the presence of NMV in the traffic stream influences PCU and saturation flow values significantly. The PCU conversion factor of any type of vehicles varies with the proportion of traffic composition. It is observed that PCU values of Bus and Utility are less than unity in few cases although they are bigger in size than the cars. This is because of their proportions in the traffic stream which are very low compared to the passenger cars. Unusually lower PCU values for instance, less than unit for buses and less than zero for Auto Rickshaw and Utility are found at some locations. The possible explanations would be their insignificant presence during the period of data collection. On the other hand, saturation flow expressed in PCU/hr is found to increase with approach width, turning radius, whereas decrease with increasing percentages of Utility. Traffic in Bangladesh consists of both motorized and non-motorized vehicles, as in many other developing countries. The static and dynamic characteristics of the different vehicles vary widely even within the same class. Also, the lack of lane discipline and unrestricted mixing of the various types of vehicle in the same right of way makes the traffic stream heterogeneous in nature. In order to study the non-lane based mixed traffic in a comprehensive manner as well as to estimate the signal design parameters, three models have been developed. The method adopted to consider non-lane disciplined traffic streams, where vehicles can occupy any position across the carriageway. Extensive field observations were made in order to formulate the mixed traffic behavior with a good approximation. Statistical analysis of traffic data obtained from both direct field measurement and video recording of traffic data enabled the estimation of parameters of the model.