The proportion of accidents caused by tired drivers varies from one study to another and is generally between about 15 and 60%. Research also shows that this type of accident is often more serious than collisions caused by other factors because reaction times are delayed and evasive manoeuvres are not taken. During the course of his research, Professor Åkerstedt, sleep researcher and professor at Karolinska Institutet and the University of Stockholm, puts tired people into a driving simulator to study how they react behind the wheel. Many of the test subjects, after first experiencing the classic symptoms of tiredness such as heavy eyelids and yawning, slip into what is known as a microsleep, when they doze off for a few seconds – often without realising it. Volvo Trucks has developed Driver Alert Support (DAS), a sensor-based system which detects when a driver is drifting off by using a camera to monitor the vehicle's course in relation to the road markings in order to detect fatigue or inattentiveness and sending out an audible warning and a visual signal. The average long-haul driver sleeps 4.6 nights a week in the cab, according to surveys carried out by Volvo Trucks. Today's driving and rest regulations for truck drivers can only improve safety if the statutory rest period really does offer proper relaxation. So Volvo has redesigned the bunk area of its sleeper cabs to help drivers sleep better, thereby keeping them more alert behind the wheel the next day. Beds are adjustable, mattresses are available in different firmness levels, and there is a choice of overlay mattresses to meet each individual driver's preferences. Available material includes stills.