BLACK TIMES - TEMPORAL DETERMINANTS OF TRANSPORT SAFETY

This paper is concerned with whether transport accident risk tends to peak at particular times, in relation to both time of day and time on task, and with the underlying causes of such peaks. Macro-analyses con firmed the presence of a clear circadian (ca 24 hour) rhythm in road a ccident risk with a major peak at ca 03:00 but suggested that this rhy thm could not be entirely accounted for in terms of drivers falling as leep at the wheel. Sleep propensity clearly shows a pronounced circadi an rhythm and performance efficiency in wakeful subjects shows a simil ar trend implying that the 03:00 road accident peak may simply reflect lowered performance capabilities. However, there are 'residual' peaks in accidents at certain times of day that are difficult to account fo r in terms of circadian rhythmicity. It is suggested that these may re flect a time on task effect which shows a pronounced, but transient, 2 -4 hour peak in risk. Only when individuals had been on duty for 12 ho urs or more did the risk exceed that found during the 2-4 hour peak. W hile an explanation for this transient peak is offered, the underlying reason for it is, as yet, uncertain and clearly warrants investigatio n in view of its practical implications. It is concluded that there ar e 'black times' when accidents are far more likely and that there is a strong need to investigate possible countermeasures. (C) 1997 Elsevie r Science Ltd.