As marijuana laws around the country face increasing scrutiny, a new study should calm some of the fears of cannabis's opponents.
Since 1985, American states that passed laws permitting medical marijuana use have seen significant reductions in the number of traffic fatalities. On average, these states had 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.
"We can also point to other characteristics such as the strength of public health laws related to driving, infrastructure characteristics, or the quality of health care systems, as a partial explanation for these findings,” Martins noted.
Any single study can be flawed, so it's important not to read too much into one paper. But these results mirror similar findings published in 2013 in the Journal of Law and Economics, which suggests they may be reliable.
Another academic article published in 2014 found what seemed to be a contrasting conclusion: Marijuana was identified in the remains of dead drivers three times more frequently in 2010 than data indicated was the norm in 1999.
This article, published in the American Journal of Epidemiology, spurred nervous headlines, including "Pot Fuels Surge in Drugged Driving Deaths" from NBC News.
But this finding is not actually strong evidence for the dangers of pot. THC, the active chemical in marijuana, can remain in a person's system for days after using, so detecting it in the blood of a driver in a crash is not evidence that they were impaired.
Furthermore, as Radley Balko, author of "Rise of the Warrior Cop: The Militarization of America's Police Forces," pointed out in a Washington Post op-ed, we should expect to find more drivers involved in crashes with marijuana in their systems, even if it has little impact on their driving ability.
"It makes sense that loosening restrictions on pot would result in a higher percentage of drivers involved in fatal traffic accidents having smoked the drug at some point over the past few days or weeks," he wrote. "You’d also expect to find that a higher percentage of churchgoers, good Samaritans and soup kitchen volunteers would have pot in their system. You’d expect a similar result among any large sampling of people. This doesn’t necessarily mean that marijuana caused or was even a contributing factor to accidents, traffic violations or fatalities."
And in fact, since Colorado legalized marijuana for recreational use in 2012, traffic fatalities have remained well below their peak in 2002. Though in the past two years Colorado has experienced an increase in traffic deaths, this rise correlated with an increase in total miles driven, suggesting that marijuana has little to do with it.
With a patchwork of different marijuana laws across the country, there should be ample data to keep researchers busy investigating the myriad public health consequences of the drug. And that's good news for people like Santaella-Tenorio, at least.
“The evidence linking medical marijuana laws and traffic fatalities lays the groundwork for future studies on specific mechanisms,” he said. “We also expect another area of study will be the association between the laws and nonfatal traffic injuries.”
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