Time and Space: The difference between following time headway and distance headway instructions

In order to guarantee safety, drivers are advised to keep large enough headways. However, headway advice is often provided in terms of time headway, without knowing whether this is the right way of presenting the advice. According to a psychophysics theory, attaining a time headway would indeed allow drivers to show higher headway choice accuracy compared to distance headway. The goal of the present study was to assess whether time headway instructions lead to more accurate headway choice compared to distance headway instructions, and whether this depends on vehicle speed and headway size. Two groups of twenty participants carried out headway instructions in a driving simulator (one time headway group and one distance headway group). Target headway size (1, 1.5 and 2 s) and vehicle speed (50, 80 and 100 km/h) were varied within participants. Absolute estimation errors (absolute difference between instructed and chosen headway) indicated a reduced accuracy for higher speeds and for larger target headways, for both time and distance headway instructions. Relative estimation errors (relative difference between instructed and chosen headways, representing a difference between under and over estimations) indicated a difference between time and distance headway instructions. Participants showed larger headways than instructed with higher speeds in case of the distance headway instructions. These results suggest that (a) time headway choice is not independent of vehicle speed thereby contradicting the predictions made by psychophysics theory; (b) relative and absolute estimation errors as the dependent variable produce dissimilar results; (c) in case of distance headway instructions, drivers choose smaller than instructed headways at higher vehicle speeds