Comparing time to collision and time headway as safety criteria

Examination of the criteria for maintaining distance between vehicles helps to better understand the behavior of following behavior and traffic flow. In this study, time headway(TH) and time to collision(TTC) criteria have been studied using driving simulator. TH is divided into two types, including braking TH, TH at the moment of considerable brake, and following TH, TH during following. The results showed that by reaching TTC to a threshold, braking TH has increased to 1.5 sec. and after this value, braking TH has remained constant. Also the comparison between braking and following TH showed that, unlike following TH, braking TH has less variance and its values did not differ significantly between lanes. That is, the driver is trying to observe a fixed amount of braking TH, 1.1 seconds, all the time, and not get closer to the front vehicle accordingly. It can be said that among the criteria, braking TH is the most important factor and considering it can be helpful in the carfollowing models.