Traffic Death Rates Fell In States That Legalized Marijuana, New Study Finds, While Those That Kept Criminalization Saw ‘Slight Increase’

States that legalized marijuana in 2016 saw meaningful declines in traffic fatalities during the years immediately following the policy change, according to a new study by Quartz Advisor. Takeaways were less clear, however, over a longer period of time that included years the report describes as “anomalies” nationwide.

Ultimately, the paper concludes, motor vehicle safety “should not be a significant concern for marijuana legalization initiatives,” especially when measured against alcohol.

“As of yet, studies have failed to show that legalization of cannabis has resulted in any significant increase in traffic fatalities in the places where it has been legalized,” it says. “However, the same cannot be said for alcohol, an intoxicant that remains legal, widely-available, and deeply ingrained in our culture.”

In states that legalized marijuana, “traffic fatalities declined or remained the same in the three years that followed, compared to a slight increase in states where it remained illegal.”

The findings, which are not peer-reviewed, examined traffic fatality data from four states that legalized adult-use cannabis in 2016: California, Maine, Massachusetts and Nevada. Quartz Advisor then compared those states’ vehicle death rates to the national average as well as to rates in five states where marijuana remained illegal during that period: Idaho, Indiana, Kansas, Nebraska and Wyoming.

In the three years following the change, the report says, none of the four legalized states saw an increase in traffic deaths. Most, in fact, saw declines.

“Three of four the four states saw a significant decrease in vehicle deaths over that span,” the paper says, “while the rate in Maine showed no change. Massachusetts saw the biggest drop, as rates fell 28.6 percent in the three years following legalization.”

Combined, the four states that legalized marijuana saw an 11.6 percent drop in traffic death rates from 2016 to 2019. That’s a sharper decline than the national average, which fell 10.6 percent over the same period.

That’s a far better outcome than in the five states the report examined where marijuana remained illegal, which together experienced a 1.7 percent increase in their combined traffic death rate from 2016 to 2019.

What might seem like a clear picture, however, gets muddier when researchers expanded the analysis to include data from 2020 and 2021, the most recent years for which National Safety Council (NSC) numbers were available. Over that period of time, the vehicle death rate in fact rose in states that legalized marijuana, although less than in the U.S. as a whole. The states where cannabis was illegal, meanwhile, saw vehicle fatality rates dip.

“Among the states that legalized marijuana in 2016, the vehicle death rate increased by 6.0 percent between 2016 and 2021,” the report says. “While this is an increase, it is slightly less of an increase than the national average, which saw a 6.2 percent increase in the traffic fatality rate over the same period. The vehicle death rate dropped by an average of 0.7 percent in the five states that have not legalized cannabis during this period.”

Why consider ignoring two entire years of data? The report explains:

“In many ways, 2020 and 2021 were anomalies, and this remains true in the vehicular accident trends. After decades of declining accident rates in the U.S., traffic fatalities picked up in 2020 and stayed high through 2021. The U.S. as a whole saw traffic fatality rates spike 18.9 percent from 2019 to 2021. States that legalized marijuana in 2016 saw a similar increase of 19.9 percent. States that have not legalized—and are notably more rural than ones that did—saw the vehicular death rate fall 2.3 percent over that period.”

“Because of this, we thought it was important to see what rates looked like with 2020 and 2021 removed from the data set,” it continues. “And it turns out, they look quite different.”

Quartz Advisor called the set of observations “interesting and nuanced—but ultimately limited.” So the publication talked to Judi Watters, public information and consumer outreach for the Maine Bureau of Insurance, who cited a Casualty Actuarial Society report from December 2022 that examined data from the U.S. and Canada from 2016 to 2019.

“The tests for the decriminalization effect on fatalities failed to detect a statistically significant change,” the 2022 report says about its U.S. findings. Similarly, the analysis “showed no statistically significant changes in the average cost per claim and claim frequency after marijuana legalization in Canada.”

The new Quartz Advisor report says that “while there is no evidence to suggest that legalizing or decriminalizing marijuana makes roads more dangerous, that is not to say that it is safe to drive while under the influence of cannabis.” It references a 2010 meta-analysis published in The American Journal of Addictions that found marijuana “causes impairment in every performance area that can be reasonably connected with safe driving of a vehicle.”

Oddly, however, that doesn’t always seem to make driving behavior commensurately more dangerous.

The 2022 Casualty Actuarial Study found that while marijuana use does affect driving, “the behavior is not always riskier; for example, slower speeds and longer following distances of impaired drivers have been reported.”

The American Journal of Addictions report includes a similar caveat:

“Surprisingly, given the alarming results of cognitive studies, most marijuana-intoxicated drivers show only modest impairments on actual road tests,” it says. “Experienced smokers who drive on a set course show almost no functional impairment under the influence of marijuana.”

Looming over popular concerns about cannabis-impaired driving is the fact that there exists no reliable test to screen for cannabis impairment specifically. Standard drug tests make it difficult or impossible to know whether someone is under the influence of marijuana or consumed it days or even weeks ago.

This summer, a report in Congress for the Transportation, Housing and Urban Development, and Related Agencies (THUD) bill said that the House Appropriations Committee “continues to support the development of an objective standard to measure marijuana impairment and a related field sobriety test to ensure highway safety.”

In February, the head of the American Trucking Association (ATA) discussed the problem with a congressional committee, arguing that lawmakers need to “step up” to address the conflict between state and federal cannabis policy as the industry faces shortages.

Tens of thousands of commercial truckers are testing positive for marijuana as part of the federally mandated screenings, data from the Department of Transportation (DOT) show.

Sen. John Hickenlooper (D-CO) sent a letter to DOT last year seeking an update on that status of a federal report into research barriers that are inhibiting the development of a standardized test for marijuana impairment on the roads. The department is required to complete the report by November under a large-scale infrastructure bill that President Joe Biden signed.

Experts and advocates have emphasized that evidence isn’t clear on the relationship between THC concentrations in blood and impairment.

A study published in 2019, for example, concluded that those who drive at the legal THC limit—which is typically between two to five nanograms of THC per milliliter of blood—were not statistically more likely to be involved in an accident compared to people who haven’t used marijuana.

Separately, the Congressional Research Service in 2019 determined that while “marijuana consumption can affect a person’s response times and motor performance … studies of the impact of marijuana consumption on a driver’s risk of being involved in a crash have produced conflicting results, with some studies finding little or no increased risk of a crash from marijuana usage.”

Another study from last year found that smoking CBD-rich marijuana had “no significant impact” on driving ability, despite the fact that all study participants exceeded the per se limit for THC in their blood.

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Photo courtesy of Carlos Gracia.