Teenagers who drink alcohol and smoke marijuana simultaneously may have a higher risk of driving unsafely, according to a recent study.

Researchers from the University of Michigan found that teens who have consumed alcohol and smoked marijuana in the past year had higher rates of traffic tickets, warnings and car accidents. Researchers found that kids who used alcohol and marijuana at the same time were at a particular risk: about 50 to 90 percent more likely to admit to unsafe driving than their peers who did not drink or smoke pot.

"It's well known that both drinking and other drug use are linked to risky driving," lead researcher Yvonne Terry-McElrath, said in a statement. "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."

For the study, researchers collected and analyzed data from surveys of more than 72,000 high school seniors in the United States, conducted yearly from 1976 to 2011 through the Monitoring the Future study, supported by the National Institute on Drug Abuse.

They found that a "significant number" of students were using both drugs in 2011, Terry-McElrath said. That included 21 percent who said they used the drugs at the same time, at least occasionally. And those kids were at heightened risk for reporting unsafe driving - even compared with their peers who only drank, and those who used alcohol and marijuana but not at the same time.

They also found that roughly 40 percent of teens who used both drugs together had received a traffic ticket or warning in the past year. And about 30 percent had been in an accident.

Researchers said it is not clear why those teenagers were at increased risk. Terry-McElrath said one possibility is that they are bigger risk takers in general. But it's also possible that using both drugs together impairs teenagers' driving - and judgment - to a greater degree.

Terry-McElrath said it's important to make kids -and the public, in general - aware of the risks of simultaneous drug use.

"Driver's education needs to talk more about the risks, in believable ways-not using inaccurate scare tactics," she said.

More broadly, she added, media messages about unsafe driving should go beyond alcohol.

"We often hear the message 'Don't drink and drive,'" Terry-McElrath noted. "But we don't hear much about the risks of using additional substances, either alone or simultaneously with alcohol."

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