The effect of time headway feedback on following behaviour

A field study was conducted to assess the impact of continuous time headway feedback on following behaviour. An equipped vehicle was fitted with a microwave radar connected to a head-down display. The display was supplemented by an auditory tone which sounded if headway decreased below 1 second. Sixteen subjects participated in five consecutive sessions conducted on a U.K. motorway. The presence of the system and the time of the journey (i.e. rush hour vs off-peak) was manipulated across the experimental sessions. Results revealed that the presence of the system reduced the proportion of time the subjects spent at low headways (e.g. < 1 second). This effect was accentuated for: (a) subjects who habitually follow at shorter headways and (b) those scenarios characterised as following a lead vehicle at a constant velocity. The presence of the system increased time headway to a lead vehicle when an overtaking manoeuvre was initiated, but only in off-peak traffic. The system had no significant effect on speed-keeping behaviour or driver's mental workload.