Empirical validation of vehicle type-dependent car-following heterogeneity from micro- and macro-viewpoints

This paper validates the vehicle type-dependent car-following heterogeneity from micro- and macro-aspects by using Next Generation Simulation (NGSIM) trajectory data. Regarding the micro-viewpoint, traffic conflict analysis techniques and three surrogate safety indexes (i.e., time headway (TH), time to collision (TTC) and safety margin (SM)) are introduced to evaluate the safety performance during ‘steady’ car following. When driving or following a larger vehicle, Z-test results show that TH, TTC and SM all become higher significantly, that is, the driver’s accepted risk tends to be lower. From the macro-viewpoint, with the aggregated traffic data, a flexible Lagrangian fundamental diagram is calibrated by a bi-level optimization procedure. These calibrated curves illustrate that during close car following process with the same speed, the driver generally keeps a higher gap distance with the leader when following or driving a larger vehicle. Therefore, the consistent findings contribute to the conclusive results about the vehicle type-dependent car-following heterogeneity.