An increasing number of Americans are driving while impaired, especially under the influence of marijuana and painkillers.

Drunk driving is a big no-no—and most people know that. But what about drugged driving? Newly released figures indicate that driving under the influence of drugs is on the rise.

Researchers at Columbia University’s Mailman School of Public Health assessed National Highway Traffic Safety Administration (NHTSA) data from 1999 to 2010 and found that 24.8 percent of 23,591 drivers who were killed within one hour of a crash were on drugs. Of them, 39.7 percent had alcohol in their systems.

During this 10-year period, the number of non-alcohol drug-related accidents rose from 16.6 percent to 28.3 percent—for marijuana alone, it went up from 4.2 percent to 12.2 percent.

The alcohol-related crashes involved a higher percentage of men than women, but the rise in marijuana-related crashes was reported for both sexes and in all age groups.

The data only include fatal car crashes in six states: California, Hawaii, Illinois, New Hampshire, Rhode Island, and West Virginia. The researchers noted that it’s possible for a driver to test positive for marijuana in a blood test up to one week after use.

Guohua Li, M.D., DrPH, a Columbia professor, said that given the increasing availability of marijuana and the ongoing prescription opioid epidemic, understanding the role of controlled substances in motor vehicle crashes is important for the health of the general public.

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According to The National Roadside Survey of Alcohol and Drug Use by Drivers from the NHTSA, one in eight weekend, nighttime drivers tested positive for illicit drugs in 2007. One in eight high school seniors responding to the 2010 Monitoring the Future Study said they had driven after smoking marijuana within two weeks of taking the survey.

A 2012 National Survey on Drug Use and Health estimated that 10.3 million people 12 years old older—or 3.9 percent of adolescents and adults—had driven under the influence of illicit drugs within a year of taking the survey. That same year, about 29.1 million people, or 11.2 percent, had driven under the influence of alcohol at least once.

Joanne Brady, a Ph.D. candidate in epidemiology and the lead author of the Columbia study, found that there was an increase in drivers testing positive for marijuana between 2007 and 2013.

She also found an increase in fatal crashes in California, where medical marijuana became legal in 1996, as well as a growing use of marijuana by patients being treated in Colorado healthcare facilities.

“The marked increase in its prevalence as reported in the present study is likely germane to the growing decriminalization of marijuana,” said Brady.

It’s hard to pinpoint exactly why there’s an increase trend in drugged driving, and those analyses warrant further investigation, but one possibility may be an increase in the use and/or access to these drugs,” added Noelle C. Anastasio, Ph.D., of the University of Texas Center for Addiction Research.

What makes marijuana so dangerous for drivers? One active ingredient—δ-9-tetrahydrocannabinol—is linked to poor driving performance, longer response times, and slower driving speeds.

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Over the past 17 years, 20 states and Washington, D.C. have enacted legislation, and four more states have legislation pending, to decriminalize marijuana for medical use.

Brady said that despite laws that prohibit drugged driving, it’s still possible that decriminalization may result in more crashes involving marijuana.

According to the Washington State Patrol, 745 motorists tested positive for marijuana during the first half of 2013. So far, marijuana is approved for recreational use only in Colorado and Washington.

“I believe that more in-depth studies on the effects of marijuana on driving and marijuana-involved crashes in those states in which it is legalized, both recreationally and medically, need to be completed,” Anastasio said.

Li said the increase in drugged driving is primarily facilitated by marijuana and opioids, the use of which nearly tripled between 1999 and 2010.

“This study did not correlate legalization of marijuana for medical use to the increase in drugged driving fatalities,” Li said, though he added that other studies have. Li said that the use of opioid painkillers has been on the rise since the early 1990s.

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Jane Metrik, Ph.D., an assistant professor at Brown University’s Center for Alcohol and Addiction Studies, said marijuana users may not be aware of its harmful effects when they get behind the wheel.

“Research shows that believing that driving after smoking marijuana is dangerous may protect people from driving when high. People who held those beliefs were less likely to drive while high,” Metrik said.

“Perceived danger and social norms are very influential in predicting high driving,” Metrik added. “There is a general trend in increased public acceptance of marijuana along with perceived lack of negative consequences from use.”

Li agrees that more people are driving drugged because they’re not aware of how harmful it can be.

“Unfortunately, this ongoing epidemic has not been widely recognized,” said Li, adding that the number of fatal crashes from drugs other than alcohol will be higher than alcohol-related fatalities by 2020 if this trend continues.

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