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.
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:
stop driving if they feel they are falling
asleep;
use highway rest stops;
drink coffee;
turn on a radio;
get at least 12 hours sleep in the 48 hours
before a trip;
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.
