American states that pass laws permitting medical marijuana use experience significant reductions in traffic fatalities, according to new study published Tuesday. On average, these states have lower rates of automobile-related deaths than states without medical marijuana laws.

Traffic fatalities fell 11 percent after enacting medical marijuana laws and had 26 percent lower rates of such deaths on average compared to other states. As of November 2016, 28 states and Washington, D.C., had legalized medical marijuana to some extent, though it remains illegal at the federal level. These findings come from a new paper published in the American Journal of Public Health from lead author Julian Santaella-Tenorio, a doctoral student in epidemiology at the Mailman School of Public Health at Columbia University.

The reductions in traffic deaths are particularly concentrated among the age group of people 15 to 44 and especially for those 25 to 44. "This finding suggests that 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," said Santaella-Tenorio in a press release.



Proponents of marijuana legalization often argue that the drug may serve as a substitute for alcohol for many users, and thus allowing marijuana may actually ameliorate some of the public health consequences of heavy drinking. Though this hypothesis is far from proven, it may help to explain the results Santaella-Tenorio and his co-authors found. "States with medical marijuana laws and lower traffic fatality rates may be related to lower levels of alcohol-impaired driving behavior in these states," said Silvia Martins, associate professor at the Mailman School and senior author of the paper. "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."



To conduct the study, the researchers used data from the Fatality Analysis Reporting System spanning the years of 1985 to 2014. During this period, there were a total of 1.22 million deaths due to automobile crashes across the U.S.

The trend of decreased traffic deaths correlating with medical marijuana legalization is robust, and the researchers also found that the opening of marijuana dispensaries had a similar effect. But the effects are not uniform: California and New Mexico, for instance, both experienced substantial reduction in traffic fatalities after passing medical marijuana but afterward saw rates climb steadily back up.

"These findings provide evidence of the heterogeneity of medical marijuana laws and indicate the need for further research on the particularities of implementing the laws at the local level," said Santaella-Tenorio. "It also indicates an interaction of medical marijuana laws with other aspects, such as stronger police enforcement, that may influence traffic fatality rates."

In other words, there are many factors that may correlate with medical marijuana laws that could impact the rate of traffic deaths.