Impact of Heavy Vehicles on Headway Distributions: Study Using Naturalistic Urban Expressway Trajectories

The headway between vehicles is an important traffic flow characteristic affecting safety, level of service, driver behavior, and capacity of a transportation system and plays an important role in the study of a broad range of problems. The primary objective of this study is to analyze the characteristics of time headways with the presence of heavy vehicles in a naturalistic urban environment. Three headway types based on different combinations of leading vehicles and following vehicles are adopted: car–car (i.e., passenger car to passenger car—C-C), car–heavy vehicle (C-HV—mainly truck), and HV-C. Accordingly, high-resolution trajectory data is extracted using videos collected at the I-395 expressway in Washington, DC, U.S.A., which comprises three main lanes and one merging lane. The headways of vehicles are categorized by lanes to assess the individual lane impact. Statistical comparisons between each vehicle type–specific headway combination is made to verify the associated car-following behavioral patterns. C-C, C-HV, and HV-C headways are found to be different and the result shows there is a statistical difference between the means. The findings indicate that the lognormal model effectively represents C-C and C-HV, as well as HV-C, headway distributions. However, in cases of severe congestion (homogeneous congestion cluster formation) in the merging lane, the gamma distribution proves to be a better fit for the C-C headway. This study indicates qualitative differences between vehicle type-specific and lane-specific headway distributions, which form the basis for microscopic traffic simulation modeling and simulation.