Kelley Blue Book: How much time do drivers spend with their eyes closed? The results of this study are scary.

A new study suggests that drivers consistently underestimate how tired they are and how much fatigue affects their driving. The findings are scary because crashes caused by tired drivers tend to be severe — nod off behind the wheel, and you won’t be alert to brake or swerve to avoid anything.

Researchers compared objective, subjective drowsiness

Sponsored by AAA, researchers at the University of Iowa asked 90 drivers to drive 150 miles on a simulated interstate highway at night after a day without caffeine or a nap. Fifty were male, 40 were female, and they averaged 31.4 years old.

Researchers asked the drivers to rate their fatigue on a 9-point scale. They then compared their subjective opinions to an objective measure — the percentage of time their eyelids spent closed.

Drivers could stop to take breaks. They could walk around, drink coffee, eat food, or use “a private room with a comfortable chair” to nap. But researchers also told them they earned a dollar for every minute under three hours they drove. Researchers didn’t pay them if they crashed or went off the road.

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Drivers declined to rest

The result? Participants significantly underestimated their drowsiness. “When drivers reported low perceived levels of drowsiness, the objective measure suggested that 75% of them were moderately or highly drowsy,” AAA says. A quarter of those whose eyes spent more than a quarter of each minute closed thought they weren’t very tired.

Few took the breaks they should have. “Even when participants rated themselves as extremely drowsy and had the opportunity to take a break, more than 75% chose to continue driving,” researchers say.

The results suggest we should assume our judgment is compromised after hours on the road.

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They also suggest automakers should work on improving driver attention monitors. Another recent AAA-backed study indicated the systems many new cars use to warn drivers to take a break aren’t very effective.

The Insurance Institute for Highway Safety – one of America’s two major safety rating agencies – plans to begin rating such systems soon.

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