THURSDAY, Dec. 13 (HealthDay News) -- Drivers often "zone out" behind the wheel, and this mind wandering contributes to many car crashes, a new study finds.

French researchers who reviewed data on nearly 1,000 collisions say half of all car accidents may be caused by a driver's wandering thoughts and worries.

"Day dreaming -- mind wandering -- accounts for a significant proportion of car crashes," said lead researcher Emmanuel Lagarde, a senior researcher with the National Institute of Health and Medical Research at the University of Bordeaux.

"It is difficult to imagine what we can do about that, because we cannot avoid thinking while driving," he said.

This study, published online Dec. 13 in the journal BMJ, is part of an effort to understand dangerous driving distractions, Lagarde said. The goal is to develop technologies that can alert drivers to hazards they may not pay attention to, he added.

External distractions, such as phoning or texting, are known to be linked with crashes. But inattention from distractions such as worries is not well understood, Lagarde said.

Researchers use "mind wandering" to describe thinking about things unrelated to what one is doing at the moment. It often happens when resting or during repetitive tasks.

All drivers occasionally let their minds drift. For some, this temporary zoning out has disastrous consequences. Taking your mind off the road may make it easier to overlook traffic hazards or to make more driving errors, Lagarde's group said.

Some people may be more prone to mind wandering than others, Lagarde said. These may include drivers with attention deficit hyperactivity disorder (ADHD) or people taking medication, he speculated.

Whether there are born mind wanderers isn't known, he added.

For the study, Lagarde's team interviewed nearly 1,000 adult drivers injured in car accidents between April 2010 and August 2011. The patients were questioned in the emergency room on average less than five hours after the crash.

The researchers asked the patients about their thoughts just before the accident, and analyzed how distracting those thoughts were.

They also took into account factors such as road and traffic conditions and whether rules of the road were followed. The drivers also had their blood alcohol level and pre-crash emotional state assessed.

Of all the accidents reviewed, 47 percent were the driver's fault, the researchers found.

In 52 percent of the crashes, drivers said their minds wandered somewhat just before the crash. Thirteen percent said their thoughts were highly disrupting or distracting -- what Lagarde called intense mind wandering.

Intense mind wandering increased driver responsibility for the accident 17 percent, compared to 9 percent of crashes where the driver was not at fault, the researchers noted.

Russ Rader, a spokesman for the Insurance Institute for Highway Safety, said distracted driving has always been a problem.

"It's not just phone calls and texting that distract drivers; they're distracted by a lot of things absent phones," Rader said.

And that's a challenge for safety advocates, he said, because laws can target and perhaps curb cell phone use but attention lapses will continue to cause collisions.

More information

For more information on distracted driving, visit the U.S. Centers for Disease Control and Prevention.