While distraction is the most common cause of crashes where a vehicle drifts out of its lane, unconsciousness or incapacitation are usually responsible for those involving serious or fatal injuries.

The Insurance Institute for Highway Safety published the results of a study of these types of crashes in its latest "Status Report" publication. Researchers found that physical issues were responsible for 47 percent of all drifting crashes, as well as 57 percent of those involving serious injuries or deaths.

The study looked at a nationally representative sample of 5,470 passenger vehicle crashes in the United States between 2005 and 2007. These crashes, logged in the National Highway Traffic Safety Administration's National Motor Vehicle Crash Causation Survey, occurred between 6 a.m. and midnight and resulted in emergency medical services being dispatched and at least one vehicle being towed from the scene.

A total of 631 incidents where a driver crashed after drifting out of their lane were analyzed for the study. IIHS says these crashes represented a national total of 259,034 crashes.

Drivers who fell asleep at the wheel accounted for 17 percent of crashes involving departure from a lane. Drivers who were incapacitated by a medical issue such as a heart attack, diabetic shock, seizure, or stroke also accounted for 17 percent of these crashes. Falling asleep and incapacitation each accounted for 21 percent of drifting crashes with serious or fatal injuries.

Drivers who were conscious but had a blood-alcohol level above the legal limit accounted for 8 percent of lane departure crashes, as well as 8 percent of those with a serious injury or death. Five percent of lane departure crashes involved other physical factors, while these factors were the cause of 7 percent of lane departure crashes with a serious injury or death.

Among drifting crashes caused by non-physical factors, 22 percent were caused by distraction. Poor control or other errors accounted for another 21 percent, while an unknown driver factor was responsible in 10 percent of these crashes.

Poor control or unknown driver factors were each present in 16 percent of drifting crashes with a death or serious injury. Distraction was a factor in 11 percent of these crashes.

While crashes where a single vehicle left the roadway made up only 21 percent of nonfatal injury crashes in 2014, they also accounted for 40 percent of fatal crashes. IIHS also found that 12 percent of fatal crashes and 10 percent of crashes involving injury involved a head-on collision or sideswipe, conditions which can be caused by a vehicle leaving its lane.

Researchers noted that one limitation of the study was the exclusion of crashes that occurred between midnight and 6 a.m. They suggested that many crashes involving drivers who fall asleep or otherwise lose consciousness likely occur during this period.

The study sought to determine how frequently drivers drift out of their lane unintentionally and cause a crash. IIHS says this data can be useful in the development of crash avoidance technology, particularly systems designed to warn drivers of a lane departure or keep them within their own lane.

Lane departure warnings track a vehicle's position on the road and warn the driver when the vehicle crosses a lane marking without the activation of a turn signal. In 2012, IIHS found that this technology would have been irrelevant in 97 percent of crashes reported to police and was not associated with a reduction in auto insurance claims. Earlier this year, IIHS researchers looked at 184 Honda vehicles brought to a dealership for servicing and found that two-thirds of their owners had turned off the lane departure warning system.

Lane keeping assist systems will automatically nudge a vehicle back between the lane markers if it starts to drift over the line. However, IIHS says this technology will have to do more to prevent a crash if a driver does not act to take control of the vehicle after the system is activated.

"If drivers are letting their vehicles drift from the lane because they are momentarily distracted, lane keeping assist could help," said Jessica Cicchino, vice president for research at IIHS and lead author of the study. "However, if drivers are physically unable to control the vehicle, it's not enough to only nudge the car back into the lane. In such cases, a crash avoidance system would need to bring the vehicle to a stop on the side of the road."

IIHS suggested that new vehicle technology may be able to prevent lane departure crashes caused by physical factors by combining lane departure warnings or lane keeping assist with in-vehicle driver monitoring. When this system determines that a driver has fallen asleep or become incapacitated, it could act to safely stop the vehicle.