Determinants of time headway adopted by truck drivers

As a measure of driving performance, time headway provides a way of representing the total interaction between a driver, his vehicle and the road environment as well as an exact description of episodes of dangerously close following. This paper presents the results of a field experiment on the effects on time headway of prolonged driving in a continuous convoy situation. In all, 12 professional truck drivers in two age groups were used. Each drove an instrumented 7 ton rigid van-type heavy goods vehicle for 11 hours on each of 4 consecutive days. Time headway was measured using a closed-circuit television system designed to provide a continuous record of instantaneous time headway. Conditions under which the drivers' following performances were measured were different types of following manoeuvre, prolonged driving and early and late shifts. Comparison with the results of an earlier study of incidental episodes of vehicle following provided an evaluation of the effects of infrequent versus continuous following. The results showed that time headways in convoy driving are much lower than in naturally occurring following situations, particularly for steady-state following during daytime hours. It also appears that drivers need a period of time to adjust to the demands of convoy driving and that driving at night requires longer time headways. No evidence of an increase in performance riskiness was found either during the 11 hour driving day or cumulatively over the 4 days of the experiment.