Preferred time-headway of highway drivers

The preferred time-headway of drivers in highway conditions is related to the likelihood of rear-end collisions. We studied traffic data from a section of southbound highway 101- a heavily commuted eight-lane freeway between San Francisco and the Silicon valley in California. We observed two parameters that drivers regulate during free flow, rush hour, and heavy traffic conditions: (1) the speed of their vehicle; and (2) the time-headway to the preceding vehicle. During free flow traffic, the preferred speeds show low variation within lanes, but large variations from lane to lane. During rush hour traffic, the time-headway between vehicles varies between 1 and 2 s for a range of traffic speeds. For all traffic conditions a lower limit of 1s is seen in time-headway, even when traffic volume does not push drivers toward tight spacing. The lower limit of 1s is consistent with what was found in several previous studies, but is significantly shorter than the 3s headway that is recommended by driving manuals. The short time-headways observed are within the limit of typical reaction time for braking by alert drivers, but probably lead to occasional accidents given variability in reaction times, decisions, and vehicle braking capabilities, especially when preview information is not available