Police talk to drivers at a DUI checkpoint last year. A study says motor vehicle crashes are up in four states where marijuana is legal. But other groups disagreed with the findings. (MARK RALSTON/AFP/Getty Images)

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According to a recent study, motor vehicle crashes are up 6 percent in four states that have legalized recreational marijuana compared with four neighboring states where the drug is restricted or illegal.

The study was conducted by the Insurance Institute for Highway Safety (IIHS) and Highway Loss Data Institute (HLDI), which say the crash data suggest that as more states legalize recreational marijuana, more effort will be needed to determine how best to prevent impaired driving crashes.

“What we’re seeing is a definite increase in crash risk that is associated with the legalized recreational use of marijuana,” says David Harkey, president of IIHS and HLDI.

Harkey cautioned that the study results indicate only a correlation between marijuana legalization and a higher number of crashes, and says more research would be necessary to determine whether marijuana use caused the increase.

Other groups disagree with the findings. They point to surveys that seem to contradict the conclusions. Experts and advocates interviewed for this report say that more study is needed and that greater education around driving impairment is a good idea.

The IIHS/HLDI study compared insurance claims for vehicle collisions in Colorado, Nevada, Oregon and Washington — where recreational marijuana is legal — with Idaho, Montana, Utah and Wyoming — where it’s not, and controlled for factors including driver age and employment status, seasonality, weather and location.

Paul Armentano, deputy director of National Organization for the Reform of Marijuana Laws, an advocacy group that works toward the legalization of recreational marijuana, said he was doubtful of the findings.

“I am skeptical that legal marijuana is the variable here that is solely responsible for the difference in these states versus controls, or that investigators could have definitively identified cannabis as a factor absent other potential confounders,” he told Consumer Reports. He pointed to demographic differences that may not be controlled for — such as tourism, population density and number of cars on the road — that he says call the findings into question.

Additionally, Armentano cited similar research that does not show a correlation between marijuana legalization and motor vehicle crash fatalities. That research includes a July 2017 study published in the American Journal of Public Health, which found no statistically significant difference in changes in motor vehicle crash fatality rates for Washington and Colorado after recreational marijuana legalization, compared with similar states where recreational marijuana is not legal.

“Even if one is to take claims regarding a slight uptick in traffic collisions at face value, this has not translated into a parallel uptick in fatal traffic crashes,” Armentano says.

Although more research may be needed to determine the effects of marijuana legalization on motor vehicle safety, Harkey says that more education is needed to alert drivers to the dangers of impairment.

“Impairment is impairment, whether it’s alcohol or marijuana or prescription drugs,” he says. “Any of those can affect your ability to drive a motor vehicle. You shouldn’t be behind the wheel if you’re impaired by any substance. That’s a message that I’m not sure is currently clearly conveyed.”

Indeed, IIHS found that drivers are largely unaware of the risks of using marijuana while driving. “There is lack of understanding about impairment risk, or impairment and driving risk, with respect to marijuana,” Harkey said. Additionally, roadside surveys of drivers in Washington show that marijuana-impaired drivers are more likely to have children in the car and are more likely to be driving during the day than alcohol-impaired drivers, IIHS said.

Another issue is the complexity of how THC, the psychoactive component of cannabis, affects driving. “Impaired driving remains a major factor in the number of motor vehicle crashes on our roads,” says Jennifer Stockburger, director of operations at Consumer Reports’ Auto Test Center. “But with marijuana, unlike alcohol, the level of driver impairment is difficult to evaluate.”

Unlike alcohol, the presence of THC in the body does not necessarily mean that an individual is impaired, and a higher level of marijuana use does not necessarily mean greater impairment. Different forms of THC-containing products affect the body in different ways. That makes it more difficult for researchers and law enforcement alike to determine impairment levels. Marijuana-impaired drivers are more likely to be impaired by alcohol as well, IIHS says, which further complicates efforts to study the problem.

“We’re going to need more research and more help from the medical community, from medical researchers, to help us understand different products with different levels of THC, how different individuals are affected by that, how that relates to impairment, and ultimately how that relates to the ability to drive a vehicle and a potential crash risk,” Harkey says.

A growing number of states are allowing some form of marijuana use. In addition to the states involved in the study, five states allow recreational use of marijuana as well as medical marijuana, more than 20 states allow medical marijuana, and more than a dozen states allow the use of certain cannabis products for medical use. Legalization initiatives are pending in multiple states, and Canada became the second country to legalize recreational marijuana in October. U.S. federal law still considers marijuana illegal to possess, use or sell.

Harkey says that road-safety advocates must now work with the marijuana industry in addition to the medical community, public-health researchers and law enforcement to reduce the number of crashes related to impaired driving.

“Are there ways we can work with the marijuana industry to try and educate the consumers and educate the public? I think these are questions for public health agencies to help us figure out,” Harkey says. He also suggests using additional tax revenue derived from marijuana sales to fund road-safety initiatives.

 Copyright 2018, Consumer Reports Inc.

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