WEDNESDAY, Dec. 21, 2016 (HealthDay News) -- Traffic deaths have fallen in many -- but not all -- states with medical marijuana laws, a new study finds.
The reason for this trend isn't clear, but the effect was most pronounced in younger drivers. That could mean that as marijuana becomes more easily accessible, more young people use it instead of alcohol, driving down drunk-driving rates, the researchers said.
"The mechanisms by which medical marijuana laws reduce traffic fatalities mostly operate in those younger adults, a group also frequently involved in alcohol-related traffic fatalities," noted study author Julian Santaella-Tenorio.
He's a doctoral student in epidemiology at Columbia University in New York City.
For the study, the Columbia team looked at 1985-2014 records from a federal government database on traffic fatalities. They found an average 11 percent reduction in such deaths in states that implemented a medical marijuana law.
Overall, traffic deaths in those states was 26 percent lower on average than in states without such laws, the team reported.
The greatest reduction in traffic deaths was among people ages 25-44, who -- according to the researchers -- account for a high percentage of medical marijuana users.
Specifically, traffic death rates fell 11 percent among those ages 15 to 24. The rates dropped 12 percent among those ages 25 to 44 and fell 9 percent among those 45 and older.
However, the study did not show that the trend always lasted.
For example, a few states actually had longer-term increases in car crash deaths after pot became legal for medical use. California had an initial reduction of 16 percent and New Mexico had an initial reduction of 17.5 percent after enacting medical marijuana laws. But, the traffic death rates in those states eventually increased again, the study found.
So the effect varied from state to state, and there may be a "need for further research on the particularities of implementing the laws at the local level," Santaella-Tenorio said in a university news release.
Other factors, including varying levels of police enforcement, might play a big role in why traffic fatalities fell in some states but not others, he said.
"Lower levels of alcohol-impaired driving behavior in these states" that enacted medical marijuana laws might play a major role as well, said study senior author Dr. Silvia Martins, an associate professor at Columbia's Mailman School of Public Health.
"We found evidence that states with the marijuana laws in place compared with those which did not, reported, on average, lower rates of drivers endorsing driving after having too many drinks," she said.
The study was published online Dec. 20 in the American Journal of Public Health.
The U.S. National Institute on Drug Abuse has more about medical marijuana.