Analysis of the relationship between speed and density in heterogeneous traffic flow on suburban highways

Over the past seventy years, several studies have been done about the relationship between macroscopic parameters of traffic flow. Based on Highway Capacity Manual (HCM), calibration of relations with parameters according with the conditions of each region is required.This study investigated the relationship between speed and density based on real data analysis for the heterogeneous traffic flow.In this regard, first the values of speed and density through each time periods calculated with a separation of line and vehicle class using data extracted from the loop detector installed in two 3-lane suburban Highways.On the other hand, the adaption of existing mixed-flow data with models Presented by Green Shields, Greenberg and Andrei Wood had been evaluated, and Results in very low adaptation rate.In the next step, using the general form of these models (linear, logarithmic, and exponential equations) we developed them for multi-class flow with considering line and day as constant parameters and density of each vehicle classes as variables.The stepwise regression procedure is used in this study as analysis method. Assumptions of regression model, such as examining the multicolinearity of variables, significant level of correlation factors, and Durbin Watson test was investigated.
At last, using the mentioned statistical tests, the best fitting model has been selected and sensitivity analysis was performed to examining different classes of vehicles affecting on speed variations of passenger car.