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Fewer drinking and driving, but marijuana use up

There's good news and there's bad news in America's fight to cut down on impaired driving.

A new study by the National Highway Traffic Safety Administration (NHTSA) found the number of drivers with alcohol in their system has dropped by nearly a third since 2007.

On the flip side, the study also found a hefty increase in the number of drivers who had marijuana or other illegal drugs in their systems. Nearly 1 in 4 drivers tested positive for at least one drug that could affect safety behind the wheel, according to NHTSA's 2013 to 2014 Roadside Survey of Alcohol and Drug Use by Drivers.

Police officer administering sobriety test
Doug Menuez | Photodisc | Getty Images

"A one-third reduction in alcohol use over just seven years shows how a focused effort and cooperation among the federal government, states and communities, law enforcement, safety advocates and industry can make an enormous difference," NHTSA's Mark Rosekind said in a news release.

"At the same time, the latest Roadside Survey raises significant questions about drug use and highway safety."

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The survey, which is voluntary and anonymous, gathers data from drivers across the country who agree to participate at a roadside site.

According to the most recent findings, about 8 percent of drivers had alcohol in their system on weekend nights. That's down by about 30 percent from the last survey, conducted in 2007, and 80 percent from the first survey in 1973.

But it wasn't all good news. The number of drivers with marijuana in their system jumped by nearly 50 percent compared to 2007.

In a separate survey, NHTSA studied whether drivers who use marijuana are more likely to be in a crash. The results found that these drivers are more likely to get in an accident; however, they are also more likely to be young men, a demographic that's already at an increased risk for traffic accidents.

This second study was conducted over a 20-month period by examining accidents involving 3,000 drivers in Virginia Beach, Virginia.

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"The combined message of these two surveys is that our work to understand and combat drunk driving is paying off, but that we have much to learn about how illegal drugs and prescription medicines affect highway safety," Rosekind said. "Developing that knowledge is urgent, because more and more drivers have these drugs in their systems."

According to the Centers for Disease Control and Prevention, more than 10,000 people in the U.S. were killed in 2012 in alcohol-impaired driving crashes. That accounts for nearly one-third of all traffic-related deaths. Drugs other than alcohol are involved in about 18 percent of motor vehicle driver deaths.

Questions? Comments? BehindTheWheel@cnbc.com.