Drowsy Driving Crash Study

Press Release

UNC Highway Safety Research Center

CHAPEL HILL, N.C. — Drivers who work night shifts, long hours, or more than one job are at increased risk for being involved in a crash caused by falling asleep at the wheel or fatigue, according to a first-of-a-kind study by the University of North Carolina Highway Safety Research Center.

Other factors strongly associated with having a drowsy driving crash include sleeping less than six hours per night, being awake for 20 hours or longer and frequent driving between midnight and 6 a.m.

"To our knowledge, this is the first study where a large number of drivers in sleep-related crashes and drivers in non-sleep-related crashes were contacted soon after the occurrence of their crash," Principal Investigator Dr. Jane Stutts of the UNC Highway Safety Research Center said. Other studies on drowsy driving have involved either surveying the general population of drivers and asking them about their experiences with drowsy driving, or analyzing police crash data from drivers involved in drowsy driving crashes.

"If you simply analyze crash data, you can learn a lot about who is involved in drowsy driving crashes and when and where they occur, but you can't really learn about why that individual fell asleep and crashed," Stutts said. "And if you simply call up drivers from the general population and ask them if they've ever fallen asleep at the wheel, you'll get quite a few who will tell you they have, but relatively few of them will have actually crashed. And those who have crashed often aren't able to give detailed information about an event that may have occurred five or ten years earlier."

In conducting the study, researchers from the Center interviewed 1,403 North Carolina drivers identified by police crash reports and driver records. These included phone interviews with 467 drivers involved in recent crashes caused by falling asleep at the wheel or being fatigued, 529 phone interviews with drivers recently involved in crashes not catalogued as fatigue- or sleep-related, and 407 phone interviews with drivers who hadn't been in a crash within the past three years.

"We found that drivers in sleep and fatigue-related crashes were 4 to 5 times more likely than drivers in the control crash group to work night-shift jobs," Stutts said. Working 60 or more hours a week also increased drivers' risk. Twenty-seven percent of the drivers involved in sleep-related crashes worked 60 or more hours a week compared to 17 percent of the drivers in the control crash group. Drivers in sleep-related crashes were nearly twice as likely as drivers in the control crash group to work more than one job.

"My impression when I talked to a lot of these people was, well no wonder you fell asleep at the wheel! You must have been just exhausted!" Stutts said.

Other risk factors examined in the study were whether drivers were using medications that may cause drowsiness at the time of their crash, and whether any had been previously diagnosed with a sleep disorder. Between 8 and 10 percent of the drivers in sleep and fatigue crashes reported taking such medications while less than 2 percent of the control crash group fell into this category. Just three percent of the drivers in sleep crashes reported having a diagnosed sleep disorder.

The study, which was funded by AAA Foundation for Traffic Safety, found that the people who hadn't been involved in crashes had considerably different profiles from those who had, Stutts said.

"Drivers who hadn't been in crashes were older and much less likely to be employed. They also were more likely to be female and reported driving less," she said. "For this reason, I tend to compare the people who were in sleep-related crashes to the people in the control crash group when talking about this study. These two populations are more alike."

Although the study shows that certain populations are at greater risk for involvement in a drowsy driving crash, Stutts cautioned that the results also show that "everyone who doesn't get enough sleep on a regular basis is at risk."

"The vast majority of people in our study who crashed as a result of driving while drowsy either got too little sleep on a routine basis and built up what sleep researchers describe as 'sleep debt,' or they got far too little sleep before trying to drive," she said. "In many cases, the people involved in these crashes were just the average 'driver next door' who happened to be putting in extra hours at work, adjusting to a new baby in the household, staying out late for a party, or trying to make it back home after an out-of-town trip."

Dr. Bradley Vaughn, director of the UNC Department of Neurology Sleep Center and a co-investigator for the study expanded on this finding. "I think we as a society don't view sleep as a necessary function but more as a luxury," he said. "The consequences of that are that in general, we try to shorten the amount of sleep we get and are more sleep deprived as a society."

The study found that sleep and fatigue crash drivers averaged over half an hour less sleep a night than the control crash group. Nearly half of them reported that they did not get enough sleep, a figure double that of the control crash drivers. They were three times as likely as control crash drivers to rate their sleep as "poor," and were more than twice as likely to suffer from extreme sleepiness during the day. More than half slept less than 6 hours the night before their crash compared to 10 percent for the control crash group. The study also indicated that many drivers do not even know how sleepy they are. Around half the drivers in sleep-related crashes said they did not feel even "moderately drowsy" before they crashed.

In examining ways that people deal with drowsy driving situations when they come up, the researchers found that the drivers involved in sleep-related crashes were much more likely to rely on behaviors that have not been shown to be effective such as opening windows or turning up the radio. Fatigue and sleep crash drivers were also less likely to plan ahead to prevent drowsiness by getting a good night's sleep before a trip or using caffeine.

"It's also noteworthy that about a quarter of the drivers in sleep crashes said that they had driven while sleepy more than ten times in the past year," Project Co-investigator Dr. Jean Wilkins of the UNC School of Medicine said. "This is not a new experience for many drivers. These people may have started to think that driving while drowsy is no big deal - they think they can handle it, that they can force themselves to stay awake. But our study shows this isn't true. Many of us would never think about driving drunk, but by driving when we're sleep- deprived, we put ourselves and others at risk of a crash as severe as an alcohol-related crash."

Media Contacts

Caroline Dickson

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