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Even though estimates from crash data 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. Like distracted driving, drivers may be reluctant to admit they dozed off following a crash. 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, 2016). 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 AAAFTS, 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). Using the number of fatal crashes in 2021 (39,508) and percentage of driver drowsiness that may have contributed to these fatal crashes (21%), this estimate suggests that more than 8,300 people may have died in drowsy-driving-related motor vehicle crashes across the United States in 2021.  

Naturalistic driving studies (NDSs) may help us understand the associations between driver drowsiness and crash risk. One recent study by the AAAFTS used the Strategic Highway Research Program 2 (SHRP2) NDS data to estimate the prevalence of driver drowsiness immediately before a crash (Owens et al., 2018). Drowsiness was assessed as the percentage of time a driver’s eyes were closed. A total of 701 crashes were examined. 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 air bag deployment, significant property damage, or injury (Owens et al., 2018).