The minimum total braking time (i.e. the braking reaction time plus the accelerator-to-brake movement time) plays an important role in defining a minimum following gap (MFG). This study was designed to obtain a lower limit for this gap. Total braking times (TBT) of a group of 51 male and female young athletes were monitored during real driving conditions. Sudden braking applied by a leading private passenger vehicle initiated the trials. A within-subject design was used to study the effects of different factors on braking time. Individuals performed a series of semi-counterbalanced trials at two following distances (6 and 12 m), two speeds (60 and 80 km/h) and three expectancy stages (naive driving, partial knowledge, and full knowledge of the forthcoming manoeuvre). A three-way repeated measures ANOVA (analysis of variance) showed no major effects of `speed', but major effects of the `expectancy' and the `distance' factors. The experiment yielded a mean TBT of 0.678 s (SD = 0.144 s) for trials averaged over distances and speeds in the naive condition only. The data emphasize the role played by pre-cues in the braking response prior to emergency stops. Both the level of awareness of the forthcoming manoeuvre and the distance between vehicles appear to determine the response time. The descriptive statistics presented may also provide the basis for an objective, acceptable and legally valid minimum time gap for prosecution of `careless' drivers. (A)
Abstract