Nearly one in seven Washington state drivers traveling with kids tested positive for THC, the psychoactive ingredient in marijuana, researchers reported Thursday.

Based on a roadside survey, the researchers determined that 14.1 percent of drivers with children on board had used cannabis, according to their report published in the Journal of Studies on Alcohol and Drugs.

“One of the things I would like consumers to know is that cannabinoid products can be impairing,” said study co-author Angela Eichelberger, a senior research scientist with the Insurance Institute for Highway Safety. “And different products and methods of ingestion might have different effects."

Another concern, she said, is that some of the people who tested positive may have been impaired.

In October, two studies found a rise in the number of highway crashes in four of the states where the recreational use of marijuana has been legalized. The studies didn't prove a cause-and-effect of marijuana use and crashes, but transportation experts are concerned about the trend.

There aren’t a lot of studies on cannabis and driving, although the existing research suggests the drug slows thinking and response time, leading to an increased risk of crashes.

The information used in the new study came from the Washington State Roadside Survey, which was conducted from June 2014 to June 2015 within six counties in the state. Data for the surveys was collected during one daytime two-hour Friday session (either 9:30 a.m. and 11:30 a.m., or 1:30 p.m. to 3:30 p.m.) and four two-hour nighttime periods in Friday and Saturday (10 p.m. to midnight, and 1 a.m. to 3 a.m.).

Drivers were invited at either a stoplight or a stop sign to turn their cars into a data collection site. They could make as much as $60 for volunteering to participate — $10 for giving a saliva sample and $50 for a blood sample. Along with the samples, drivers filled out a questionnaire.

Of the 2,056 Washington drivers who opted to participate, 238, or 9.3 percent, were accompanied by a child. The good news, the researchers say, is that campaigns about drinking and driving seem to have resonated.

The news wasn’t as good when it came to marijuana.

The likelihood that a motorist would test positive for THC did not appear to be altered by the presence of a child in the car: Fourteen percent traveling with a child were positive for THC, compared to 17 percent of those who were not accompanied by a child. The difference was not statistically significant, Eichelberger said, meaning it could just be due to chance.

The survey founds that motorists' attitudes about marijuana affected whether they used it and then drove with a child. Among those who thought cannabis was “very likely” to impair driving, 8.9 percent tested positive for THC. That’s compared to 40.6 percent of those who thought that it was “not very likely at all” to impair driving.

The new study highlights the fact that many people don’t realize cannabis can be impair driving ability, said Marilyn Huestis, a professor at The Lambert Center for the Study of Medicinal Cannabis and Hemp at Thomas Jefferson University.

While there are national standards for alcohol consumption — someone with a blood alcohol content of 0.08 percent or higher is considered cognitively impaired and unsafe to drive — there's not yet a clear policy on when someone is too high to drive.

“The truth is, if everything goes as it’s supposed to go, you can make it home,” Huestis said. “But you can’t respond appropriately and quickly when an unexpected event occurs. You see this over and over again in crash cases.”

The new findings are “worrisome,” said Dr. Katherine Hoops, an assistant professor of pediatric care medicine at the Johns Hopkins Children’s Center. “This is shedding light on a troubling public health problem, especially when you consider that motor vehicle collisions are a leading cause of death in children in the U.S.”