Two new studies suggest that marijuana legalization could lead to more traffic fatalities, though the researchers behind each analysis caution that there is no proof that any of the reported traffic deaths were caused by cannabis use.

The first study, Association of Recreational Cannabis Laws in Colorado and Washington State With Changes in Traffic Fatalities, 2005-2017, published in JAMA Internal Medicine, focused on Colorado and Washington to determine whether traffic fatalities increased following the start of legal cannabis sales in each state in 2014. The researchers reviewed data on traffic fatalities over 12 years between 2005 and 2017, sourced from the Fatality Analysis Reporting System.

They found that legal adult-use marijuana sales in Colorado were associated with 75 excess traffic fatalities per year, but that traffic-related deaths in Washington stayed much the same.

“We observed that recreational cannabis laws were associated with increases in traffic fatalities in Colorado but not in Washington state, [perhaps due to] the size of the marijuana industry in Colorado, evidence of cannabis tourism in Colorado and other local aspects,” said study co-author Julian Santaella-Tenorio.

The second study, Change in Traffic Fatality Rates in the First 4 States to Legalize Recreational Marijuana, also published in JAMA Internal Medicine, considered the experiences of Oregon and Alaska in addition to Colorado and Washington. Oregon legalized adult-use marijuana in 2014 through a voter ballot initiative but marijuana dispensaries didn’t open for business till the following year. Alaska also voted to legalize in 2014 and sales began in 2016.

The researchers included 20 other states where marijuana remains illegal to serve as controls for their analysis. Like the first study, the researchers analyzed two full years of traffic fatality data, taken from the National Highway Traffic Safety Administration, following the start of recreational cannabis sales. They began their analysis of all the states from 2008.

The team of researchers found an increase of 2 deaths for every billion miles driven, compared with those without legal cannabis. From their analysis, they extrapolated that nationwide cannabis legalization would lead to 6,800 excess traffic-related deaths per year across the country.

However, the study’s authors acknowledge that “[I]t is possible that another confounder, rather than marijuana legalization and commercialization, caused the observed increase in roadway deaths.”

Indeed, neither study is able to say marijuana intoxication played a part in any of the traffic fatalities included in their analysis. Even a postmortem toxicology report that detected marijuana in a driver’s system could be the result of consumption several days prior, meaning it does not conclusively prove impairment at the time of an accident.

Last year, a congressional research body pointed to such difficulties in attributing traffic accidents to marijuana use. While marijuana DUI laws are in force across the country, there is still little consensus on how to measure and regulate marijuana intoxication. That said, Oklahoma appears to be learning by doing and is set to become one of the first states in the country to implement a roadside cannabis breathalyzer test.

The authors of both studies acknowledge the shortcomings in their research, with a JAMA journal editorial for the most recent edition calling for more rigorous research on the issue.

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