A new study finds a higher percentage of high school seniors say they have driven after using marijuana than after having five or more alcoholic drinks.

The study found 12 percent of seniors said they drove after using marijuana in 2011, compared with 10 percent in 2008. Overall, 28 percent of high school seniors said they had ridden in a car in the previous two weeks with a driver who had used drugs or alcohol, or said they had driven after using drugs or alcohol themselves, NBC News reports.

The findings, published in the American Journal of Public Health, come from the Monitoring the Future project, which each year surveys approximately 17,000 high school seniors. The increase in marijuana use among teen drivers was seen throughout the country.

“It’s a big deal… the sheer numbers,” University of Michigan researcher Patrick O’Malley told NBC News. “It has been increasing steadily and looking down the road, it seems likely to get worse. We are concerned.”

The teens were not asked about how much marijuana they smoked, or who they drove with, the article notes. The researchers said it is not known how much marijuana impairs a person’s driving. “We don’t have any good degree of impairment,” O’Malley said. “It’s almost impossible to say what the level of marijuana in your system is.”