ATLANTA, Georgia (CNN) — Driving while under the influence of marijuana and other drugs is on the rise in the United States and could be involved in a large number of fatal accidents, a new report finds.

The percentage of drivers who tested positive for marijuana or illegal drugs rose from 12.4 percent in 2007 to 15.1 percent in 2013 and 2014, according to a report by the Governors Highway Safety Association, an advocacy group that promotes traffic safety.

The data came from voluntary roadside surveys by the National Highway Traffic Safety Administration, which collected blood and saliva samples from drivers.

The report also noted that 38 percent of the people who died in automobile accidents in 2013 and who were tested had detectable levels of potentially impairing drugs, both illegal and legal, in their system. That is nearly the same percentage as tested positive for alcohol.

The most common drugs were marijuana (34.7 percent) and amphetamines (9.7 percent), a class of stimulants that includes ADHD medications and nasal decongestants. The National Highway Traffic Safety Administration data did not specify the type of amphetamine.

"Alcohol-impaired driving is still a big deal, but we have paid more attention to it than to drug-impaired driving and it's time to pay more attention to drug-impaired driving," said James Hedlund, an independent researcher and author of the report, which describes the problem of drug-impaired driving and makes state-level recommendations.

Although there has been less drunk driving over the last few decades, drugged driving appears to be increasing, Hedlund said.

A couple obvious reasons for this rise could be that "marijuana use is increasing, driven in parts by the states that legalized marijuana for medicinal and recreational use, and the second is that prescription pain killer use has gone up substantially," Hedlund said.

The report concludes that marijuana and most illegal drugs could double a driver's risk of crashing. However, it is still "highly debatable" how much drugs actually increase crash risk because study findings have been all over the place, Hedlund says.

Research suggests that the dangers of driving while intoxicated are irrefutable. Studies have found that the crash risk is two times higher in drivers with a blood-alcohol concentration (BAC) of 0.07 to 0.09, and it shoots up to four to times higher in those with BAC of 0.1.

"Alcohol is the deadliest drug we have by practically any metric...and alcohol in combination with [marijuana] is particularly malignant," said Dr. Gary Reisfield, professor of psychiatry at the University of Florida.

While alcohol tends to make drivers go faster, marijuana slows their driving speed and their reaction time, Reisfield said. And studies have found that amphetamines can make drivers speed up and pay less attention to the road. "Impaired driving can manifest in dramatically different ways, depending on the impairing drug," he said.

Recommendation

One of the recommendations of the Governors Highway Safety Association report for reducing drugged drivers is to train law enforcement officers to recognize the types of impairment associated with different substances, Hedlund said. Although there is no easy breath test to detect marijuana or other drugs, as there is for alcohol, officers could learn the physical and behavioral signs to look out for, either on the road or in the station.

Currently, 15 states have zero tolerance laws for at least one potentially impairing drug. There are 18 states with either zero tolerance laws for driving with marijuana or that set limits on the legal level. Some of the states, such as Alaska and Oregon, that have decriminalized the recreational use of marijuana have no laws.

"Every state should look at [creating] laws...it's useful for all states because marijuana is not just confined to states where it is legal," Hedlund said.

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