MODELING FUNDAMENTAL DIAGRAM WITH EMPIRICAL TRAFFIC DATA UNDER DIFFERENT ROAD GEOMETRY AND TRAFFIC OPERATING CONDITIONS

Abstract

Fundamental diagram (FD), a graphical representation of the relation among traffic flow, speed, and density, has been the foundation of traffic flow theory, hence contributes to understanding transportation engineering for many years. For example, the analysis of traffic flow dynamics relies on input from this FD to find development, propagation and dissipation of congestion. Moreover, traffic engineers can also use the FD to determine the traffic flow characteristics so that a highway and its facility can be evaluated over time and space. Underlying these importance, FD corresponding to relation between speed and density; which roughly correlates to drivers’ speed choices under varying car-following distances, is used for this study. Since reported traffic stream models are mainly developed for homogeneous traffic conditions, they may not be directly suitable for the traffic condition of Bangladesh which shows weak lane discipline and heterogeneous in nature. From broad literature, hardly any studies have been found to focus this issue especially in Bangladesh. Only a very few studies have been reported from India, which is not sufficient to justifiably represent the traffic scenario in Bangladesh. In this situation, the present study investigates the characteristics of speed-density FD i.e. its shape and structure, for different roadway geometry and traffic operating conditions comprises both non-lane-based and heterogeneous traffic of Bangladesh. It also investigates the flow parameters for stated traffic condition. For this study, 6 different cases have been considered. Traffic data is collected from 12 different locations of 3 major highways of Bangladesh. Data collection is done through video recording which is further processed by image processing technique. To investigate the structure of FD, the speed-density plots of field data for different locations are fitted with six established FD models namely: linear, 2nd degree polynomial, 3rd degree polynomial, exponential, 2nd degree exponential and logarithmic. From this investigation, it reveals that 2nd degree exponential structure is the best fitted FD model for most of the cases under prevailing traffic condition. It is also found that, free-flow speed tends to decrease dramatically if there is no footpath or shoulder and presence of market along the road-side. It is also found that if the free-flow speed decrease, the jam density is increased.