When is a driver likely to have a sleep-related accident according to researchers?

Sleep laboratories around the world are finding that an alarming number of drivers on motorways may be falling asleep at the wheel. Although researchers have difficulty in knowing for certain whether an accident has been caused by sleepiness, it appears that a driver who is on the road between 4 a. m. and 6 a. m. is about 10 times as likely to have a sleep-related accident as someone who is driving in the middle of the morning or early in the evening. Some British police forces have become sufficiently concerned to launch campaigns to alert the public to the danger. Major motor manufacturers such as Ford and Renault are investigating ways of incorporating sleepiness detectors and alarms into their vehicles.
    However, British government bodies responsible for road safety have not initiated any studies into the problem of sleepy drivers on motorways. The Department of Transport claims that it is "aware of the problems" , but does not regard it as a high-priority issue and is not planning to support any relevant research apart from a general study on " driver behaviour".
    Unfortunately, the issue is clouded by the fact that many motorways accidents that might be caused by sleepiness are categorized under other headings, such as "inattention" , "failed to look or see other vehicle" and "misjudged speed/distance". Figures collected in the 1970s by the Transport and Road Research Laboratory list the cause of 20 percent of all road accidents as "perceptual errors". "Fatigue" was specified in only 2 percent of cases. However, few investigators inquire further to discover just why a driver was not attending, failed to look or made errors in perception. For various reasons, including the fear of prosecution and possible difficulties with insurance claims, drivers are reluctant to admit to falling asleep, but are more willing to admit to "inattention". When these rather vague responses are examined thoroughly, sleepiness often emerges as the true culprit.
    Driving on a road as dull as on a motorway makes worse sleepiness in a driver who is already sleepy. But how can we tell if an accident on a motorway has been caused by sleepiness? There are some strong pointers. If an accident involves only one vehicle, which runs off the road into the central crash barrier, a tree or a bridge, then sleepiness is likely to be the cause, especially if there are no skid marks or other signs of braking. A driver who is alert to impending crash grips the steering wheel and suffers different injuries from someone who is asleep and holding the steering wheel loosely. This pattern of injury, combined with an absence of skid marks on the road, also suggests that the driver was asleep in accidents where one vehicle runs into the back of another, especially if it occurs where traffic is light and vehicles are consequently well-spaced on the road.