A new study from the AAA Foundation for Traffic Safety explores the relationship between drowsy driving and motor vehicle crashes — finding 9.5% of drivers involved in crashes to be classified as drowsy. Of the 589 crashes studied, 56 drivers were rated as drowsy over the three minutes immediately prior to the crash.

Drowsiness was identified in 8.8% to 9.5% of all crashes examined and 10.6% to 10.8% of crashes that resulted in significant property damage, airbag deployment, or injury.

The study — "Prevalence of Drowsy Driving Crashes: Estimates from a Large-Scale Naturalistic Driving Study" — also found that a high proportion of crashes (54.4%) involved drivers under the age of 25, though this population was deliberately oversampled.

Drowsiness varied significantly in relation to lighting conditions. For example, drowsiness was evident in over three times the proportion of crashes in darkness as in daylight. Moreover, half of the crashes that involved drowsiness occurred in darkness.

The study draws on data from the federally funded Second Strategic Highway Research Program Naturalistic Driving Study.

It assesses drowsiness in 3,593 drivers using a validated method — the PERCLOS measure, which is the percentage of time that a person’s eyes are closed over a defined measurement period. Drivers from six sites around the U.S. were monitored continually via in-vehicle cameras and other data collection between October of 2010 and December of 2013.

While official U.S. government statistics indicate that only 1% to 2% of all crashes involve drowsy driving, this new study suggests the problem may be a far greater.

Read the full study here.