Drivers who get behind the wheel stoned instead of drunk may actually be making the roads safer in states that allow medical marijuana, according to new research.

Economists Daniel Rees of the University of Colorado Denver and Mark Anderson of Montana State University looked at traffic fatalities in thirteen states that enacted medical marijuana laws between 1990 and 2009.  They found that on average, traffic fatalities in those states fell nearly 9 percent after medical pot became legal.

“What’s going on is that young adults– especially males– were drinking less when medical marijuana became legal,” Rees tells ABC News, pointing to data from the Beer Institute that showed a drop in beer sales in states with new medical marijuana laws.  “You legalize medical marijuana and the highways become safer.”

Why?  Rees and Anderson have two theories.

“One hypothesis is that it’s just safer to drive under the influence of marijuana than it is drunk,” Rees says.  “Drunk drivers take more risk, they tend to go faster.  They don’t realize how impaired they are.  People who are under the influence of marijuana drive slower, they don’t take as many risks.”

The other theory, Rees says, is that people smoking marijuana simply don’t go out as much.

Could other factors be at work?  For example, some states like Tennessee and Virginia, have seen declines in traffic fatalities since 1994 even without medical marijuana laws.   And in Colorado–where medical marijuana is legal–police have seen increasing numbers of stoned drivers.  In 2010, 32 people involved in fatal crashes had ingested marijuana, according to the Colorado Department of Transportation.

Rees says he and Anderson stand by their research, which they note has not yet been peer-reviewed by colleagues.  They say they carefully accounted for nationwide trends and other policy changes — such as seat belt laws or lower speed limits– that could also be responsible for lowering traffic deaths.

“It’s really hard to think, once you’ve accounted for all those things, what could be reducing alcohol consumption and be correlated with legalization of medical marijuana,” Rees said.