A growing percentage of teens do not see marijuana use as a distraction while driving, and nearly one in five (19%) say they have gotten behind the wheel after smoking pot, a study reported Wednesday. Thirteen percent of teens report driving under the influence of alcohol.

In the study of nearly 2,300 11th- and 12th-graders across the country, commissioned by Liberty Mutual Insurance and SADD, 70% of teens say marijuana use is "very" or "extremely" distracting to their driving, down from 78% in 2009.

The findings reflect "a dangerous trend toward the acceptance of marijuana and other substances compared to our study of teens conducted just two years ago," says Stephen Wallace, senior adviser for policy, research and education at SADD (Students Against Destructive Decisions).

The trend has been reflected elsewhere. In December, the University of Michigan's "Monitoring the Future" study of 47,000 eighth-, 10th- and 12th-graders found that marijuana use rose in 2011 for the fourth straight year and that daily use was at a 30-year peak level among high school seniors.
Smoking and driving.

The new study's findings are disturbing "both in terms of the increased use of marijuana and from the perspective that many think this is not a danger," Wallace says.

Among teens who have driven after using marijuana, 36% say it presents no distraction when operating a vehicle. Nearly one in five (19%) say alcohol is no distraction.

In talking with teenagers around the country, "we hear from young people who believe that marijuana actually makes them a safer driver, that they concentrate harder, drive slower," Wallace says. Those are all misconceptions, he says, adding that "marijuana affects memory, judgment, and perception" and can lead to poor decisions when behind the wheel.

The study highlights the need "to get the message out about the dangers of marijuana impairment," says Tom Hedrick of The Partnership at Drugfree.org, an advocacy group that was not involved in the study. "It's a wake-up call for parents about the importance of having this conversation" with their teens.

Friends and peers also play a role in getting teens to make safer driving choices, Wallace says. He notes that 72% of teen passengers said they would speak up and ask a driver who used marijuana to not drive; 87% would ask a driver who had been drinking. Says Wallace, "We've got to empower passengers to speak up."

Copyright 2012 USA Today, a division of Gannett Co. Inc.