Annually, drowsy driving accounts for approximately 328,000 crashes, 109,000 injuries, and 6400 fatalities. It is estimated that 7.6% of all fatal crashes from 2017-2021 involved a drowsy driver.
Although many drowsy driving crashes are caused by a driver experiencing a microsleep or actually falling asleep at the wheel, accidents caused by sleep deprivation are likely not the result of decreased alertness alone. That is because, in addition to drowsiness, the effects of sleep deprivation include:
Drowsy driving research has found that time of day is the single most important factor contributing to fatigue and reduced alertness when driving and is the single best predictor of crashes. In fact, drowsiness episodes are 8 times more likely between midnight and 6am than during other times of the day. One study found that drivers are 20 times more likely to fall asleep while driving at 6am than at 10am. Additional sleep-related factors that may contribute to fatigue include how much total sleep a driver has gotten, what time the sleep occurred, and the quality of the sleep. In addition, environmental conditions (e.g., darkness, monotonous driving environment), underlying medical conditions (e.g., sleep apnea) and drug use (e.g., methamphetamine, drugs that cause drowsiness) may also contribute to fatigue.
Human Factors investigations of drowsy driving crashes are scientific investigations that consider the underlying causes of sleep deprivation and fatigue to answer two main questions:
Was the driver fatigued at the time of the collision?
Was fatigue a cause of the collision?
A human factors analysis includes evaluating the driver’s sleep/wake history, circadian factors, and health factors to determine the extent of sleep loss/deprivation as well as evaluating the driver’s actions to determine whether a driver was exhibiting the known effects of fatigue or sleep deprivation at the time of the collision.