In the literature many definitions are used for fatigue. The concepts of "fatigue", "sleepiness" and "drowsiness" are often used interchangeably. Sleepiness can be defined as the neuro-biological need to sleep, resulting from physiological wake and sleep drives. Fatigue has from the beginning been associated with physical labour, or in modern terms task performance. Although the causes of fatigue and sleepiness may be different, the effects of sleepiness and fatigue are very much the same, namely a decrease in mental and physical performance capacity.
The most general factors that cause fatigue are lack of sleep, bad quality sleep and sleep demands induced by the internal body clock. Besides these general factors, prolonged driving (time-on-task) can increase driver fatigue, especially when drivers do not take sufficient breaks. For specific groups of drivers, e.g. professional drivers, these general factors often play a more persistent role due to long or irregular work schedules. A small part of the general population (3-5%) has to cope with obstructive sleep apnoea, a sleeping disorder which contributes to above average day-to-day sleepiness.
Fatigue leads to a deterioration of driving performance, manifesting itself in slower reaction time, diminished steering performance, lesser ability to keep distance to the car in front, and increased tendency to mentally withdraw from the driving task. The withdrawal of attention and cognitive processing capacity from the driving task is not a conscious, well-planned decision, but a semi-autonomic mental process of which drivers may be only dimly aware. Drivers may try to compensate for the influence of fatigue, for instance by either increasing the task demands (e.g. driving faster so that a 'new' sensation of driving spurs adrenaline and attention levels) or lowering them (e.g. increasing the safety margins by slowing down or using larger following distances). But crashes and observations of driving performance show that compensatory strategies are not sufficient to remove all excess risk.
Survey research world-wide suggests that over half of all private drivers drive while being fatigued or drowsy at least once a year. Amongst young drivers, driving while fatigued is quite common due to lifestyle factors. Adolescents need more sleep than adults; fatigue may affect youngsters more than adults. Most professional drivers and shift workers have to cope with fatigued driving on a frequent basis due to work-related factors. About half of professional drivers take less than normal sleep time before a long-distance trip.
Fatigue is a major factor in a large proportion of road crashes (range 10-20%). Several studies suggest that fatigue is associated with increased crash risk. A person who drives after being awake for 17 hours has a risk of crashing equivalent to being at the 0.05 blood alcohol level (i.e. twice the normal risk). The increased risk often results from a combination of biological, lifestyle-, and work-related factors. More scientific evidence is needed concerning the exact quantitative relationship between fatigue and risk.
Driver fatigue countermeasures may be directed at drivers, transport companies, roads or vehicles. Drivers may learn how to prevent driver fatigue by campaigns. Transport companies can introduce special policies to educate both drivers and management about the problem. Roads may be equipped with edgelines or centrelines that provide audio-tactile feedback when crossed over. In the future, legislation concerning working and rest hours may be further improved and vehicles can be equipped with devices that detect fatigue-related decrements in driver performance.