Driver Drowsiness and Fatigue in the Safe Operation of Vehicles - Definition of Terms and Concepts

The terms “drowsiness” and “fatigue” are used interchangeably as drowsy driver and driver fatigue; however, they are defined very differently. There is not an agreed upon operational definition of fatigue, and, even today, it remains unclear. However, generally speaking, fatigue is usually defined as a state of reduced physical or mental alertness that impairs performance and is often the result of physical or mental exertion (Williamson et al., 1996); whereas drowsiness is the inclination to sleep resulting from lack of sleep, monotony and boredom, hunger, or other outside factors (Stutts et al., 1999). Drowsiness and fatigue are difficult to objectively measure; therefore, it is challenging to assess and consequently regulate how to avoid driving while fatigued. As fatigue or hypovigilance are difficult to assess via post-crash reconstruction, estimates of the frequency and outcomes of fatigued driving are likely conservative. According to a consensus statement published by a group of experts in fatigue and transportation, fatigue contributes to 15 to 20% of transportation crashes (Akerstedt et al., 2000). Moreover, drivers may be hesitant to disclose their level of drowsiness while driving or unaware of the fluctuations of their alertness. The severity of a crash may also leave a driver too incapacitated to report this information. Nevertheless, it is clear that fatigued drivers are more likely to be involved in crashes and are more likely to be involved in higher severity crashes as their reaction times are often delayed (National Academies of Science, Engineering, and Medicine, 2016). Thus, there is a need for approaches that measure drowsiness and fatigue as it relates to the safe operation of a vehicle. This document provides a set of concepts and definitions of terms that can be used to measure drowsiness and fatigue.