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mise à jour du
19 décembre 2002
 Injury Prevention
Drowsiness, counter-measures to drowsiness, and
the risk of a motor vehicle crash
P Cummings, T D Koepsell, J M Moffat, F P Rivara
Harborview Injury Prevention and Research Center and the Department of Epidemiology, University of Washington, Seattle, WA, USA
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An estimated one fourth to one half of drivers report having fallen asleep at the wheel at least once. Studies in the United States have estimated that between 1% and 4% of crashes may be attributed to the driver falling asleep or being drowsy. Studies from Norway, Australia, and Britain have given estimates of 4%, 6%, and 16% respectively. These studies have usually been case series in which investigators assumed that drivers or police could determine which crashes were caused by drowsiness. None sampled the prevalence of drowsiness among drivers not in crashes. They also assumed that crashes could be ascribed to just one or two causal factors, rather than a more complex combination. Admonishing people not to drive while drowsy may be too non-specific to be an effective message. More helpful advice might be generated from knowledge of how crash riskchanges in relation to specific measures ofdrowsiness that drivers can assess and modify. We conducted a case-control study to estimatehow drowsiness related factors were related tothe risk of a crash. [...]
Discussion : Our study population can be thought of as English speaking adult drivers with listed telephone numbers who could be reached at home with relative ease. Our estimates could be biased if subject selection was influenced by drowsiness related exposures.We tried to minimize this potential bias by applying the same selection scheme to both case and control drivers. We were probably less likely to sample drivers who spent a great deal of time traveling, because they may have been less accessible for a telephone interview.We had evidence that we were less likely to interview potential case drivers who crashed at night, had been drinking, orwere issued a citation; similar information regarding potential control drivers was not available. Furthermore, our findings may not apply to drivers in urban areas or to crashes in which the driver dies.

Differential recall between case and control drivers could have biased some estimates. In particular, case drivers, seeking to explain why they crashed, might be more likely to recall a feeling of falling asleep on their journey, particularly if their crash was related to actually falling asleep. By whatever mechanism, a feeling of falling asleep was associated with a greatly increased risk of a crash. A study of 28 sleep deprived drivers using a driving simulator reported that subjects were always aware of increasing sleepiness before having a simulatedcrash due to falling asleep. The aRR of a crash increased exponentially with the distance driven, when we comparedtrips of equal duration. Once a trip was more than 130 miles, the increase in aRR associated with any further increment in distance was greater than expected if the increase was simply proportional to the distance driven. For example, if we compared a drive of 600 miles with one of 300, the relative risk of a crash increased more than the twofold increase in distance: 2.2(6Ð3) = 10.6. This may be consistent with the theory that a driver may become progressively less alert.

However, our estimates regarding miles driven, independent of driving time, may also reflect vehicle speed;we had no measure of vehicle speed at crash or reference time. Naps were uncommon in our study, and were more often taken by the drivers who crashed; a nap may indicate an unsuccessful attempt to fight sleepiness. Using a highway rest stop, however, was associated with a decreased risk of crashing; notably, most of these breaks did not involve napping.

Drivers who had on a radio or tape player, were at less risk for a crash compared with others. This is consistent with a study of 16 sleepdeprived subjects in a driving simulator; when they played a radio or tape the subjects had fewer driving errors, although the diVerence was not statistically significant. The same study found no improvement in performance when cold air was blown in the subject's face; this agrees with our finding that having a window open was not related to the risk of a crash. Our finding that drinking coVee was associated with a decreased crash risk is consistent with studies in which 150 mg of caVeine was administered to sleep deprived subjects who were in driving simulators.30 CoVee contains about 17 mg of caVeine per ounce compared with about 4 mg in soft drinks.31 This difference could explain our failure to find evidence of a protective effect from sodas.

We expected that yawning might indicate drowsiness; contrary to this expectation, drivers who recalled yawning were at decreased risk of a crash. We were also surprised that the drivers who slept the most during the previous 48 hours had an increased risk of crashing compared with those who slept 12 hours. Perhaps this reflected longer term sleep deficits that were not fully corrected, or it could be a marker for people who need more sleep. Lastly, we cannot explain why drivers awake for 12 to 17 hours might have a lower crash risk compared with drivers awake for shorter or longer periods. These unexpected results were not produced by our modeling choices, as different category boundaries and nonparametric regression revealed the same patterns. Chance associations in our data or biases in exposure measurement might explain these findings; alternatively, they may offer clues to causal mechanisms that are not apparent to us.

In a previous study, North Carolina drivers in crashes that law oYcers attributed to sleepiness were compared with drivers in other crashes, and with other drivers not in a recent crash. As the authors noted, this study estimated the relative risk of being in a sleep related crash compared with being in a crash not attributed to sleepiness, or the relative risk of being in a crash classified as sleep related compared with not crashing. These estimates are not comparable with our estimates for the relative risk of crashing compared with not crashing.

A case-control study from Spain examined the association between sleep apnea and the risk of a traffic crash among 102 drivers who crashed and 152 control drivers. Drivers with an apnea-hypopnea index of 10 or higher, compared with those with a lower index, had an aRR for crashing of 7.2 (95% CI 2.4 to 21.8). Case drivers, compared with controls, had slightly higher mean Epworth scores and snoring scores, although these diVerences were not statistically significant.

figure 1
Implications for prevention

To our knowledge, this is the first study to provide relative risk estimates for a crash, derived from an actual driving population, for nearly all of the potentially hazardous and protective factors that we examined. Our findings oVer some evidence that drivers on rural highways may decrease their risk of a crash if they:

  1. stop driving if they feel they are falling asleep;
  2. use highway rest stops;
  3. drink coffee;
  4. turn on a radio;
  5. get at least 12 hours sleep in the 48 hours before a trip;
  6. avoid driving long distances, such as 300 miles, by sharing the driving or interrupting the trip.
Our data also support the construction of convenient highway rest stops.
Monotony of road environment and driver fatigue: a simulator study

Thiffault P, Bergeron J.

Laboratoire de Simulation de Conduite, Center de Recherche sur les Transports, Universite de Montreal, C.P. 6128, Succursale Centre-Ville, Que, H3C-3J7, Montreal, Canada
Accid Anal Prev 2003 May;35(3):381-91

Studies have shown that drowsiness and hypovigilance frequently occur during highway driving and that they may have serious implications in terms of accident causation. This paper focuses on the task induced factors that are involved in the development of these phenomena. A driving simulator study was conducted in order to evaluate the impact of the monotony of roadside visual stimulation using a steering wheel movement (SWM) analysis procedure. Fifty-six male subjects each drove during two different 40-min periods. In one case, roadside visual stimuli were essentially repetitive and monotonous, while in the other one, the environment contained disparate visual elements aiming to disrupt monotony without changing road geometry. Subject's driving performance was compared across these conditions in order to determine whether disruptions of monotony can have a positive effect and help alleviate driver fatigue. Results reveal an early time-on-task effect on driving performance for both driving periods and more frequent large SWM when driving in the more monotonous road environment, which implies greater fatigue and vigilance decrements. Implications in terms of environmental countermeasures for driver fatigue are discussed.