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Drowsy driving is a prevalent safety concern that results in large part from lifestyle patterns and choices. In 2018 there were 775 fatalities involving drowsy drivers, representing 2.1% of all motor vehicle traffic crash fatalities (NCSA, 2019). Drowsy driving was reportedly involved in 2.1% to 2.5% of fatal crashes (NHTSA - FARS, 2014-2018) and 1.8% to 2.2% of overall injury crashes from 2016 to 2018 (NHTSA - CRSS, 2016-2018). Until recently, attention and research on drowsiness has primarily been concentrated on commercial truck drivers, but the problem is far more widespread. In a 2006 survey by the National Sleep Foundation, 51% of 552 young drivers (grades 10 to 12) reported driving at least once while drowsy (NSF, 2006). In 2014 more than one-third of 444,306 respondents 18 and older in the United States reported sleeping less than 7 hours a day—the minimum sleep duration recommended for optimal well-being (Liu, 2016). The 2017 AAA Traffic Safety Culture Index found that more than 40% of 2,613 drivers reported getting less than 6 hours of sleep a night in a typical week (AAAFTS, 2018).

Another study by the AAA Foundation aimed to quantify the relationship between sleep deprivation and crash risk (Tefft, 2016). A sample of 4,571 crashes that occurred from July 2005 to December 2007 in the United States was analyzed using a case-control design. Elevated crash risk was associated with fewer number of hours of sleep in the 24 hours before a crash. Drivers who reported less than 4 hours, 4-5 hours, 5-6 hours, and 6-7 hours of sleep were associated with a 11.5, 4.3, 1.9, and 1.3 times increase in crash rate than drivers who reported sleeping at least 7 of the past 24 hours. In addition, the study also estimated that changes from their normal sleep patterns can elevate drivers’ crash risk. Drivers who slept 1-2 hours, 2-3 hours, 3-4 hours, or 4+ hours less than usual had crash rates that were 1.3, 3.0, 2.1, and 10.2 times greater than drivers who reported sleeping as usual. Overall, drivers who averaged 4 to 5 hours of sleep daily were estimated to have 5.4 times the crash rate of drivers who averaged seven or more hours of sleep (Tefft, 2016).

A change in the social perception of sleep and public awareness about the risks of drowsy driving are necessary to change a driver’s decisions to drive when drowsy (Higgins et al., 2017). Few behavioral highway safety countermeasures have been shown to reduce drowsy driving, although new countermeasures are currently being developed and evaluated. NHTSA hosted the Asleep at the Wheel: A Nation of Drowsy Drivers forum in 2015 with the objective of addressing drowsy driving by bringing together the traffic safety and the sleep science communities (NHTSA, 2016a; Higgins et al., 2017). NHTSA released the Drowsy Driving Research and Program Plan in 2016 describing projects related to quantifying the problem, building public awareness and education, developing policy, identifying high-risk populations, and advancing vehicle technology and infrastructure (NHTSA, 2016b; Higgins et al., 2017). The National Transportation Safety Board reported that about 36 major transportation investigations from 2001 to 2012 identified fatigue as a potential cause or contributing factor; this included approximately 14 highway incident investigations (Marcus & Rosekind, 2017; GHSA, 2016). NTSB added “human fatigue” as an issue in its “Most Wanted List of Transportation Safety Improvements” in 2016 (GHSA, 2016; NTSB, 2016).

Problem size and characteristics

Drowsy driving can cause drivers to be less responsive to driving events in a way that potentially increases the risk of crashing (Lyznicki et al., 1998). Similar to distracted driving, most drivers acknowledge drowsy driving is potentially dangerous, but many still engage in this behavior. Several studies across the past two decades have estimated the portion of the population who have fallen asleep at the wheel through self-reporting. NHTSA surveyed 4,010 drivers in spring 2002 and found 11% reported that they had nodded off while driving during the past year (Royal, 2003). Of those who nodded off, 66% said they had 6 or fewer hours of sleep the previous night. A 2010 survey of 2,000 U.S. residents found 41% of drivers reported having ever fallen asleep or nodding off while driving (AAAFTS, 2010). Four percent of drivers reported falling asleep while driving in the past month, while 11% had done so in the past year. A similar, more recent study found that nearly all drivers (97%) believe it is unacceptable to drive while excessively drowsy, yet 32% admitted to having driven while too tired to easily keep their eyes open in the past 30 days (AAAFTS, 2016; similar results from the 2017 Traffic Safety Culture Index are reported in AAAFTS, 2018). A CDC analysis of the Behavioral Risk Factor Surveillance System phone survey across 19 States and the District of Columbia found that about 4% of 147,076 respondents reported falling asleep while driving at least once in the past 30 days (Wheaton et al., 2013). This proportion was consistent with a different sample of over 90,000 U.S. residents surveyed in 2011-2012 (Wheaton et al., 2014). A meta-analysis of seventeen international studies published between 1993 and 2014 reported that drivers who self-reported experiencing sleepiness while driving were at more than twice the risk of being involved in a motor vehicle crash compared to drivers who reported no such instances (Bioulac et al., 2017).

These surveys provide additional useful information about drowsy driving. Three of the studies found that young drivers and male drivers were more likely than older drivers and female drivers to have dozed off at the wheel (AAAFTS, 2010; Wheaton et al., 2013; Wheaton et al., 2014; Royal, 2003). Moreover, driving while drowsy does not just occur late at night. About one-quarter of those drivers who admit to nodding off say the most recent incident occurred in the afternoon (noon to 5 p.m.), which might be attributable to circadian rhythms (Royal, 2003). Drowsy driving is also not limited to long trips – roughly half of the drivers who nodded off had been driving for an hour or less. Adults who reported sleeping less than 6 hours per day, snoring, or unintentionally falling asleep during the day were more likely to fall asleep while driving (Wheaton et al., 2013). Drowsy driving was also found to be more prevalent among binge drinkers and those who seldom or never used seatbelts (Wheaton et al., 2014), which suggests that drowsy driving may be more prevalent among drivers who generally engage in riskier behaviors. A driving simulator study conducted in Australia identified another vulnerable group of fatigue-prone drivers—regular commuters (Caponecchia & Williamson, 2018). Participants with sleep deprivation were worse at lane keeping than participants with no sleep deprivation with longer durations of sleep deprivation producing greater lane deviations. The effect was increased in the morning even on short drives, and the authors suggest that sleep inertia could be a potential factor. Sleep inertia is a transitional phase between sleep and wakefulness that results in low arousal and impaired performance (Tassi & Muzet, 2000; Caponecchia & Williamson, 2018).

Even though post-hoc estimates provide a useful picture of the risk of driver drowsiness (Tefft, 2016), it is often difficult to determine whether drowsy driving contributed to a crash. Similar to distracted driving, drivers may be reluctant to admit they dozed off following a crash. Current data estimates that 2% to 20% of annual traffic deaths are attributable to driver drowsiness, according to the NHTSA Drowsy Driving Research and Program Plan (NHTSA, 2016b). According to NHTSA’s Fatality Analysis Reporting System (FARS) and the Crash Report Sampling System, (CRSS) annually from 2016 to 2018, there were on average over 96,000 police-reported crashes involving drowsy drivers. These crashes injured more than 52,000 people and killed more than 800. However, researchers have inferred the existence of additional drowsy-driving crashes by looking for correlations with related factors such as the number of passengers in the vehicle, crash time and day of week, sex of the driver, and crash type. A study by the AAA Foundation, using data from 1999 to 2013, found that driver drowsiness may have contributed to 6% of all crashes and 21% of fatal crashes (Tefft, 2014). This estimate suggests that more than 6,000 people may have died in drowsy-driving-related motor vehicle crashes across the United States last year.

Naturalistic driving studies (NDSs) may help us understand the associations between driver drowsiness and crash risk. One recent study by the AAA Foundation for Traffic Safety used the Strategic Highway Research Program 2 (SHRP2) NDS data to estimate the prevalence of driver drowsiness immediately before crashes (Owens et al., 2018). Drowsiness was assessed as the percent of time a driver’s eyes were closed (Wierwille et al., 1994). It examined 701 crashes. Estimates show that drowsiness may have been a contributing factor in 8.8% to 9.5% of all crashes examined and in 10.6% to 10.8% of crashes that resulted in airbag deployment, significant property damage, or injury (Owens et al., 2018).