Analysis of the Time Headway Distributions under Day and Night Conditions in Heavy Traffic Flow

The aim of this study is to analyze the time headway distributions of vehicles on multi-lane highways under heavy traffic flow conditions. In this study, the driver behavior was assessed when selecting headways at daytime and nighttime peak hours. The field-data was collected at the Shahid Kharrazi six-lane highway in Isfahan, which was under uninterrupted flow conditions. The primary survey shows that a high percentage of drivers do not consider safe headways; but instead use a car-following process, which leads to an increasing risk-ability of drivers and causes a safety reduction. The chi-squared method was used to test the goodness of fit of the statistical models on headway distributions, and the results show that the lognormal model with a shift of 0.24 seconds is an appropriate model for headways of the passing lane. In addition, the gamma distribution with a shift of 0.69 seconds was a good fit to headway data collected from the middle lane. The dissimilarity of headway models is due to different behaviors of drivers in the lanes during traffic peak hours; as the pattern of car-following in the passing lane is different to that in the middle lane. The majority of drivers in the passing lane tend to follow the vehicles in front even at high speeds. This has triggered the difference in capacity. In addition, the results obtained from a comparison of models during day and night hours show that at high levels of flow rate in night hours, the drivers choose headways that are safer than during daytime hours.