People who combine alcohol and marijuana often drive afterward

September 20, 2022

A third of drivers who drink alcohol and use marijuana at the same time report getting behind the wheel within two hours of consumption, a new study from the Insurance Institute for Highway Safety shows. That’s a concern because evidence suggests the combination of alcohol and marijuana worsens driving performance more than either substance by itself.

Only 1 in 10 drivers surveyed by IIHS reported using alcohol and marijuana simultaneously over the past year, compared with 7 in 10 who reported past-year drinking. But among those simultaneous users, 33 percent reported drinking alcohol and consuming marijuana and then getting behind the wheel within two hours.

“Alcohol impairment is associated with almost 30 percent of the fatalities on our roadways, and we have made very little progress to reduce this toll over the past three decades. Now we are adding another impairing substance to the mix,” IIHS President David Harkey said ahead of his presentation at the annual meeting of the Governors’ Highway Safety Association in Louisville, Kentucky. “Policymakers, law enforcement, safety professionals and others will need to work together and implement multiple solutions to save the thousands of lives cut short every year by impaired driving in the United States.”

Experimental studies suggest using alcohol and marijuana simultaneously worsens driving performance more than either substance used separately. An earlier IIHS study of patients who were at a hospital emergency room as the result of a motor vehicle crash only showed an increased crash risk associated with marijuana when it was combined with alcohol. Recent research shows that young adults are more likely to drive or take other risks after using both substances than after consuming marijuana alone. But while legalization is associated with increases in the number of people who sometimes drink alcohol and sometimes use marijuana, it’s unclear from the National Survey on Drug Use and Health how frequently they’re consuming the two substances simultaneously.

To begin exploring that question, IIHS surveyed more than 3,000 adult drivers across Connecticut, Delaware, New York, North Carolina, South Carolina and Virginia about their opinions and behaviors related to alcohol, marijuana and driving. Connecticut, New York and Virginia all legalized marijuana for recreational use in 2021, while it remains illegal in Delaware, North Carolina and South Carolina.

Overall, 71 percent of drivers in the six states reported past-year drinking, 24 percent reported past-year marijuana use and 11 percent reported past-year simultaneous use of both substances. Among the drivers who reported drinking alcohol, 28 percent reported drinking two hours or less before driving. Among the drivers who reported using marijuana, 41 percent reported using the drug two hours or less before driving. Among those who reported simultaneous use, 33 percent reported doing so two hours or less before driving.

Past-year alcohol use was lowest among drivers ages 18-20 and highest among those 30-39, while past-year marijuana use was lowest among drivers 70 and older and highest among drivers ages 21-29. Self-reports of driving within two hours of using alcohol, marijuana or both followed the same broad pattern.

Men were more likely than women to use either substance before getting behind the wheel. About 25 percent of men reported drinking and driving, compared with 15 percent of women. Twelve percent of men reported using marijuana and 5 percent reported using both marijuana and alcohol before driving, compared with 8 percent and 3 percent of women.

By state, greater percentages of New York and Connecticut drivers reported past-year use of alcohol, marijuana and both simultaneously than drivers in the other four states. Similarly, drivers in New York were the most likely to agree that marijuana-impaired driving is a problem in their area. Perhaps because Connecticut, New York and Virginia adopted their new laws less than a year before the survey was conducted, most of the responses did not split clearly between states where recreational use is legal and states where it is prohibited. For example, the lowest percentages of drivers in North Carolina (illegal) and Virginia (legal) reported past-year marijuana use.

“We don’t have enough data yet to draw conclusions about how the legal status of marijuana may be affecting how people are using it,” said Senior Research Scientist Angela Eichelberger, lead author of the paper. “We’re working on that question now to see if we can identify any clear trends.”

While roughly two-thirds of drivers in all the states agreed that drinking and driving is a problem in their community, opinions about marijuana use and driving varied by state. Similarly, a substantially greater portion of drivers ages 50 and older than drivers ages 18-29 agreed that marijuana increases crash risk, while at least 90 percent of drivers from all age groups agreed that alcohol makes driving more dangerous.

“Those results show that there’s more work ahead to educate the driving public about the increased risk of impairment associated with alcohol, marijuana and the combination of the two substances,” Harkey said.

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