The passage of state medical-marijuana laws is associated with a subsequent drop in the rate of traffic fatalities, according to a newly released study by two university professors.

The study — by University of Colorado Denver professor Daniel Rees and Montana State University professor D. Mark Anderson — found that the traffic-death rate drops by nearly 9 percent in states after they legalize marijuana for medical use. The researchers arrived at that figure, Rees said, after controlling for other variables such as changes in traffic laws, seat-belt usage and miles driven. The study stops short of saying the medical-marijuana laws cause the drop in traffic deaths.

“We were pretty surprised that they went down,” Rees said Tuesday.

The study was posted this month on the website of the Bonn, Germany- based Institute for the Study of Labor and has not yet been peer-reviewed.

Rees said the main reason for the drop appears to be that medical-marijuana laws mean young people spend less time drinking and more time smoking cannabis. Legalization of medical marijuana, the researchers report, is associated with a 12-percent drop in the alcohol-related fatal-crash rate and a 19-percent decrease in the fatality rate of people in their 20s, according to the study.

The study also found that medical- marijuana legalization is associated with a drop in beer sales.

“The result that comes through again and again and again is (that) young adults . . . drink less when marijuana is legalized and traffic fatalities go down,” Rees said.

The study is sure to add fuel to a debate over the impacts of Colorado’s medical-marijuana boom on traffic safety, which has embroiled cannabis advocates and law enforcement officials for more than a year.

The state legislature this year rejected a bill that would have set a threshold of THC — the psychoactive chemical in marijuana — that would qualify someone as too stoned to drive. After more research and a fractious debate, the Colorado Commission on Criminal and Juvenile Justice will not recommend that the legislature try again with such a bill this year.

“The working group was not able to come to consensus,” said Arapahoe County Sheriff Grayson Robinson, who led the subcommittee that studied the issue.

Much of that debate has focused on marijuana’s impact on an individual’s driving abilities. Rees and Anderson say their study does not mean it is safer to drive stoned than drunk. Instead, they write, increased medical-marijuana usage at home might change patterns of substance use and driving.

Mason Tvert, the head of the pro- marijuana-legalization group SAFER, said the study suggests legalizing marijuana would be beneficial in unexpected ways.

“People who are drinking drive faster, take more risks, underestimate how impaired they are,” he said.

John Ingold: 303-954-1068 or jingold@denverpost.com