Modeling of time headway distributions in day and night conditions under heavy traffic flow

The aim of this study is to analyze the time headway distributions of vehicles at multi-lane highways under heavy traffic flow conditions. In this study, the driver behavior in choosing headways at daytime and nighttime peak hours was assessed. The field-data were collected at Shahid Kharrazi six-lane highway in Isfahan. The highway was under uninterrupted flow conditions. The primary survey shows that a high percentage of drivers do not consider safe headways through car-following process. It leads in increasing risk-ability of drivers which causes safety reduction. Using chi-square tests the goodness of fit of the statistical models on headway distributions were examined. The results show that the lognormal model with shift of 0.24 seconds is an appropriate model for headways of the passing lane. While, the gamma distribution with shift of 0.69 seconds was fitted on headway data collected from middle lane. The dissimilarity of headway models in is because of different behaviors of drivers in the lanes .in the traffic peak hours; the pattern of car-following in passing lane is different of that in middle lane. Majority of drivers of passing lane tend to follow the front vehicles even at high speeds. This has triggered difference in capacity. Also, the results obtained from comparison of models in day and night hours show that at high levels of flow rate at night hours, the drivers choose headways safer rather than daytime hours.