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The number of American drivers killed in car crashes in which drugs were detected has eclipsed those killed in crashes where only alcohol was found, according to a new study released Wednesday.

The report by the Governors Highway Safety Association and the Foundation for Advancing Alcohol Responsibility, a nonprofit funded by alcohol distillers, found drugs of all types — illegal and prescription — were present in 43 percent of fatal crashes in 2015 in which test results were available, compared to around 37 percent who tested positive for alcohol.

Carol Akers' son Jacob, 22, was killed by a driver high on opiods in Tennessee in 2014.

"The physical pain you eventually heal from, but the emotional pain, just never," Akers told The Tennessean newspaper last year.

The governors association calls for increased training for law enforcement to detect drugged drivers. Unlike a Breathalyzer test to detect drunk driving, police say there is no standard roadside test to detect most drugs.

Related: Marijuana Use Doubles in U.S., But So Do Problems

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The California Highway Patrol has increased training and by the end of the year expects all its officers to be trained to detect drugs other than alcohol when stopping suspected impaired drivers, CHP Sgt. Tony Garrett said.

"When it comes to driving under the influence of some type of drugs, there’s a huge difference" from alcohol, he said. In the past marijuana was easily detectable by smell, but some cannabis-infused products no longer depend on smoking the drug, he said.

"It can be extremely difficult if you don't know what you’re looking for," he said.

The data in the report has limitations. The foundation only collected what states report, and states vary in how often tests are used and what substances are tested, the report says. Nine states tested 85 percent or more of fatally-injured drivers in 2015, while two states tested 15 percent or less, the report says.

And the data only records the presence of drugs, not the amount of the drug that could be used to compare to an equivalent blood-alcohol level, the report cautioned. Many impaired drivers are combining substances, which can be especially dangerous, Ralph. S. Blackman, President and CEO of the Foundation for Advancing Alcohol Responsibility, said.

"Drugged driving is a complicated issue. The more we can synthesize the latest research and share what’s going on around the country to address drug-impaired driving, the better positioned states will be to prevent it," study author Dr. Jim Hedlund, a former National Highway Transportation Safety Administration official, said.

From 2014: Pot Fuels Surge in Drugged Driving

The effect of marijuana laws allowing medicinal or recreational use is unclear. The study released this week cites a 2013 study that found increases in marijuana use in fatal crashes in only three of 14 states that passed medical marijuana laws before 2010.

It also cited a 2016 study from the Rocky Mountain High Intensity Drug Trafficking Area that said traffic deaths involving drivers who tested positive for marijuana rose from 10 percent in 2009 to 21 percent in 2015, but those numbers include any time marijuana is detected, and other substances could be involved. Colorado voters approved recreational marijuana in 2012.

Mary Gaston's 23-year-old son, Blake, was killed in a crash in Washington state in 2013 after his motorcycle was hit by a driver who was under the influence of marijuana. "I can still hear the impact," she said.

"There's a perception that marijuana does not impair you like alcohol does. That you can smoke a couple joints, eat a couple of brownies and go out and drive. That's not true, you are impaired."

The driver in the crash, Caleb Floyd , was sentenced to 34 months in prison in October of 2015, with credit for the around 17 months he'd already served, the Seattle Times reported.