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Studies Find Crash Rates Rise When States Legalize Marijuana; Suggest Link to Alcohol

Researchers report increasing evidence is that motor vehicle crash rates rise when states legalize recreational use and retail sales of marijuana, although what effects marijuana by itself has on drivers remains unclear.

Crash rates spiked with the legalization of recreational marijuana use and retail sales in California, Colorado, Nevada, Oregon and Washington, new studies by two insurer backed organizations, the Insurance Institute for Highway Safety (IIHS) and the affiliated Highway Loss Data Institute (HLDI) show.

However, the preliminary results of a separate IIHS study of injured drivers who visited emergency rooms in California, Colorado and Oregon showed that drivers who used marijuana alone were no more likely to be involved in crashes than drivers who hadn’t used the drug. That is consistent with a 2015 study by the National Highway Traffic Safety Administration that found that a positive test for marijuana was not associated with increased risk of being involved in a police-reported crash.

“Our latest research makes it clear that legalizing marijuana for recreational use does increase overall crash rates,” says IIHS-HLDI President David Harkey. “That’s obviously something policymakers and safety professionals will need to address as more states move to liberalize their laws — even if the way marijuana affects crash risk for individual drivers remains uncertain.”

More than a third of U.S. states have legalized recreational marijuana for adults 21 and older. The hefty tax revenues those states are earning have others exploring similar legislation, and recent polls indicate that 68 percent of American adults favor legalization. Consumption also appears to be expanding rapidly, with self-reports of past-month marijuana use doubling from 6 percent to 12 percent of those surveyed between 2008 and 2019.

Road safety advocates say that’s a potential concern. Driving simulator tests have shown that drivers who are high on marijuana react more slowly, find it harder to pay attention, have more difficulty maintaining their car’s position in the lane and make more errors when something goes wrong than they do when they’re sober. But such tests have also shown marijuana-impaired drivers are likely to drive at slower speeds, make fewer attempts to overtake and keep more distance between their vehicle and the one ahead of them.

To better understand the net impact on safety, researchers at IIHS and HLDI have conducted a series of studies since 2014 examining how legalization has affected crash rates and insurance claims in the first states to legalize recreational use.

Rates Jumped

The most recent of these studies from IIHS shows that injury and fatal crash rates in California, Colorado, Nevada, Oregon and Washington jumped in the months following the relaxation of marijuana laws in each state.

Combined, the impact of legalization and, subsequently, retail sales in the five states resulted in a 6 percent increase in injury crash rates and a 4 percent increase in fatal crash rates compared with other Western states where recreational marijuana use was illegal during the study period. Only the increase in injury crash rates was statistically significant.

That’s consistent with a 2018 IIHS study of police-reported crashes — most of which did not involve injuries or fatalities — that found that legalization of retail sales in Colorado, Oregon and Washington was associated with a 5 percent higher crash rate compared with the neighboring control states.

Insurance records show a similar increase in claims under collision coverage, which pays for damage to an at-fault, insured driver’s own vehicle, HLDI’s latest analysis shows. The legalization of retail sales in Colorado, Nevada, Oregon and Washington was associated with a 4 percent increase in collision claim frequency compared with the other Western states over 2012-19. That’s down slightly from the 6 percent increase HLDI identified in a previous study, which covered 2012-18.

Uncertain Effect

Despite those increases in crash rates, studies of whether marijuana itself makes drivers more likely to crash have been inconsistent. The latest one from IIHS — which used data collected from injured drivers in three emergency rooms in Denver, Colorado; Portland, Oregon; and Sacramento, California — showed no increased crash risk associated with the drug, except when combined with alcohol.

Researchers conducted surveys for more than a year, interviewing and drug-testing more than 1,200 patients in total. The results showed that the crash-involved drivers weren’t any more likely to self-report or test positive for marijuana alone than other drivers who weren’t involved in a crash and were at the emergency room for reasons other than an injury.

Just 4 percent of the drivers involved in crashes self-reported marijuana by itself over the previous eight hours, compared with 9 percent of those who weren’t involved in a crash. Similarly, 13 percent of the crash-involved drivers tested positive for marijuana only, compared with 16 percent of the control set.

The reverse was true for the combined use of marijuana and alcohol, with 3 percent of the crash-involved drivers and fewer than 1 percent of the control drivers self-reporting use of both substances and 5 percent of the crash-involved drivers and fewer than 1 percent of the control drivers testing positive.

Those combined-use numbers could help explain why crash rates have increased. Legalization may be encouraging more people to drink and use marijuana together.

Studies comparing the simultaneous use of alcohol and marijuana in states where marijuana is legal with states where it is still against the law will be needed to test this hypothesis. But some early evidence has already emerged that shows self-reports of past-month marijuana and alcohol use have increased, while the reported use of alcohol alone has decreased, especially in states where recreational use of marijuana is now legal.

A nationally representative survey conducted recently by the AAA Foundation for Traffic Safety also found that drivers who self-reported using both alcohol and marijuana were more likely than those who had only consumed alcohol to say they had driven while impaired and engaged in dangerous driving behaviors such as making aggressive maneuvers or speeding on residential streets.

Other factors related to how legalization has affected the way people use marijuana, rather than the physiological effects of the drug, may also be at play. For example, the larger spike in crash rates in Colorado — the first state to legalize recreational use — suggests a burst of enthusiasm that leveled off as the drug’s new status became more commonplace. The first few states to legalize marijuana even used the legalization as part of their tourism promotions.

It’s also possible that disparities in state and local regulations might be encouraging more travel by marijuana users. For example, marijuana users in counties that do not allow retail sales may drive to counties that do. Their increased travel could lead to more crashes even if their crash risk per mile traveled is no higher than that of other drivers.

Source: Insurance Institute for Highway Safety (

This article was first published in Insurance Journal.

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