A new study finds teenagers who use marijuana and alcohol together are more likely to engage in unsafe driving, compared with those who use one of those substances alone.

“Simultaneous use makes a big difference in your risk for unsafe driving,” said lead researcher Yvonne Terry-McElrath of the University of Michigan’s Institute for Social Research. “There’s a very clear increase in risk for this group of kids, and for the rest of us on the roads.”

Teens who used alcohol alone were 40 percent more likely to admit they had gotten a traffic ticket and 24 percent more likely to admit involvement in a traffic crash, compared with teens who didn’t smoke marijuana or drink. Teens who smoked marijuana and drank were 90 percent more likely to get a ticket and 50 percent more likely to be in a car crash, compared with their peers who didn’t use either substance.

The findings appear in the Journal of Studies on Alcohol and Drugs.

“As you go from alcohol use to alcohol and marijuana use concurrently, your risk increased,” Terry-McElrath told HealthDay. “There’s a clear stair-stepping up the risk process for tickets and warnings.”

The researchers used data from the Monitoring the Future study, which includes surveys of more than 72,000 high school seniors, the article notes.

“It’s well known that both drinking and other drug use are linked to risky driving,” Terry-McElrath said in a news release. “But this suggests that it’s not only the frequency of substance use that’s important. The patterns of drug use are also related to the risk of unsafe driving.”

She noted that teens who use alcohol and marijuana together may be bigger risk takers in general. It’s also possible that using both substances together impairs driving and judgment to a greater degree than using either substance alone, she said.