By Amy Norton
THURSDAY, April 25, 2019 (HealthDay News) -- Many people may drive with marijuana in their system -- even when they have kids in the car.
That's the upshot of a new study of drivers in Washington state, where recreational pot is legal.
In roadside tests of more than 2,000 drivers, researchers found that 14% of those with a child in the car tested positive for THC, the component that creates marijuana's "high."
In contrast, only 0.2% of people driving with a child tested positive for alcohol on breath tests. None had levels above the legal limit.
Researchers stressed, however, that people with THC in their systems were not necessarily driving while stoned.
A positive THC test simply means the person has recently used pot.
Still, the possibility that some of those drivers were impaired is concerning, said study co-author Angela Eichelberger, a researcher with the Insurance Institute for Highway Safety.
It's not clear how often marijuana use contributes to traffic accidents, according to Eichelberger, because it's a tricky question to study. In the "real world," she noted, drivers who test positive for THC often have used other substances, too. They also tend to be young, which is in itself a risk factor for crashes.
But, Eichelberger said, controlled lab studies have shown that marijuana interferes with driving skills.
"Cannabis can be impairing," she said, though there is "no consensus" on what is the impairment threshold. How much can a person consume before it's unsafe to drive? How long should you wait to get behind the wheel after using marijuana?
The form in which people use marijuana also matters: The effects of edible products take longer to set in compared to smoking, Eichelberger noted.
For now, she said, it makes sense for people to avoid driving if they are feeling any effects from their marijuana use. She also suggested people "err on the side of caution," and make sure they will not be driving anytime soon after using the drug.
J.T. Griffin is chief government affairs officer for the advocacy group Mothers Against Drunk Driving. He was not involved with the study.
"I think the big, remaining question is: Are these people driving while impaired?" Griffin said.
With more and more states legalizing marijuana for medical or recreational use, it will be important to answer some fundamental questions, according to Griffin.
"We'll need to figure out what marijuana impairment looks like -- and how to test for it," he said.
Griffin agreed that people who are feeling the effects of marijuana should not get behind the wheel. He pointed to the latest safety campaign from the U.S. National Highway Traffic Safety Administration, which is targeting not only drunken driving but high driving. The slogan is: "If you feel different, you drive different."
The findings, published online April 24 in the Journal of Studies on Alcohol and Drugs, are based on 2,056 drivers in Washington state. Researchers approached the drivers at stop lights and asked them to participate. Volunteers gave breath, saliva and blood samples.
Overall, drivers were much more likely to test positive for THC than alcohol: Of those driving without kids in the car, almost 18% tested positive for THC, while 4.5% had alcohol in their systems.
Of adults who were driving with children, 0.2% tested positive for some amount of alcohol, while 14% tested positive for THC.
The upshot, according to Eichelberger, was that having children in the car seemed to deter drinking. But it didn't make a statistical difference in the likelihood of detecting THC.
Washington voters approved legalizing marijuana for recreational use in 2012. As other states consider the same, Eichelberger said, they might want to weigh the possible impact on impaired driving.
Other research, she said, has found a "small increase" in crash rates in states that have legalized recreational marijuana.