The perceived criticality of different time headways is depending on velocity

Time headway (THW) presumably represents a velocity-independent measure of safe following distances during car-following. However, studies using on-road data show that drivers decrease their THWs as velocity increases (e.g., Brackstone, Waterson, & McDonald, 2009). This contradicts recent findings of simulator-based studies suggesting independence of velocity and chosen THW (Siebert, Oehl, & Pfister, 2014; Siebert, Oehl, Bersch, & Pfister, 2017). This study aimed at investigating the relationship between velocity and subjective risk of different THWs using an experimental setup with a motion-based driving simulator. N = 29 participants followed a preceding vehicle with five different velocities ranging from 30 km/h to 140 km/h in urban, rural and highway environments. At each velocity level, drivers were instructed to follow with three different THWs (0.7 s, 1.1 s and 1.5 s). Subjective criticality ratings were analysed for the different combinations of these independent variables (IV1: velocity, IV2: THW). Drivers rated situations with short THWs as more critical when following with slower velocities compared to higher velocities. These results show that the criticality of theoretically velocity-independent THWs is indeed depending on velocity. These findings have both theoretical implications, such as a better understanding of drivers' car following behavior and risk acceptance, and practical implications as they may be used in the design of automated driving functions.