Evaluating the Time Headway Distributions in Congested Highways

Time headway is a significant traffic flow parameter that affects the capacity and safety of highways and freeways. Time headways are broadly used in different areas of traffic and transport engineering such as capacity analysis, safety studies, car following and lane changing behavior modeling, and level of service evaluation. In this paper, the time headway distribution is investigated for an urban highway at different traffic flow rates during congestion. To analyze the headway characteristics, the time headways to the preceding and following vehicles are analyzed for heavy vehicles and passenger cars. The trajectory data used in this study was provided for a highway section in California: Berkeley Highway, I-80. Appropriate models of headway distribution are selected for heavy vehicles and passenger cars using Chi-Square test. Using the selected models, headway distributions are predicted for each vehicle type at different traffic flow rates. The results confirm existence of different time headway distributions in vicinity of heavy vehicles and passenger cars which is due to the difference in the behavior of drivers in vicinity of heavy vehicles and passenger cars under congestion. 