A combination of marijuana and alcohol can increase the risk of unsafe driving by nearly 90 percent in teens, a new study finds.

This is one of the few studies that look into the "simultaneous use" of alcohol and marijuana. The study was conducted by University of Michigan researchers who looked into data of surveys carried out yearly from 1976 to 2011 through the Monitoring the Future study. Respondents of the surveys included 72,000 U.S. high school seniors.

Researchers found that the participants who had used alcohol and marijuana in the past year had higher rates of traffic tickets/warnings and car accidents. They also found that teens who used both the substances together were 50 to 90 percent more likely to indulge in unsafe driving.

"It's well known that both drinking and other drug use are linked to risky driving," said lead researcher Yvonne Terry-McElrath of the University of Michigan's Institute for Social Research in Ann Arbor, in a press 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."

Terry-McElrath and her team found that 40 percent of teens who used both marijuana and alcohol at the same time got at least one traffic ticket or warning in the past year. And about 30 percent were involved in an accident.

Researchers were not able to determine the exact reason behind this mechanism but speculate that it could be because simultaneous use of marijuana and alcohol impairs a teen's driving and judgment to a greater degree.

Though marijuana and alcohol use among teenagers has dropped since 2011, there is still a "significant" number of young adults that uses these substances. Researchers noted that in 2011, 33.3 percent high school seniors said they hadn't used either drug in the past year, compared to only 12 percent in 1979.

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

Another study conducted last week found that smoking marijuana increases the risk of heart problems in young adults.  A similar study conducted in September last year found that drivers who test positive for drugs are three times more likely to be in fatal car accidents and this risk increases 23 times for drivers who test positive for both drugs and alcohol.

The new study was published in the Journal of Studies on Alcohol and Drugs and supported by the National Institute on Drug Abuse.