Many teen drivers believe it's less dangerous to drive after smoking marijuana than after drinking alcohol, a perception the government wants to change.

"Driving sober means no alcohol, no marijuana, no drugs," John Walters, the Bush administration's drug policy director, said Thursday as he showed a new television ad aimed at stopping teens from driving after smoking pot.

Walters' office is spending $10 million on the ad and other efforts to teach teens and their parents about the danger of drugged driving. There also are brochures that are being distributed in high schools and state motor vehicle offices.

Marijuana can affect concentration, perception and reaction time up to 24 hours after it's smoked, Walters said. Yet teens have gotten the message that it's a benign drug.

In a recent study, 30 percent of teens said "planning to drive" was a reason not to drink. But only 18 percent cited "planning to drive" as a reason not to take drugs. The survey questioned 3,574 middle and high school students nationwide in spring and was conducted for Students Against Destructive Decisions and Liberty Mutual Insurance.

A 2004 study of patients admitted to the trauma unit at the University of Maryland found that 19 percent of crash victims under 18 tested positive for marijuana.

Allison Whitney, 25, a drug counselor and recovering addict from Atlanta, said she got into several accidents as a teenager because she was smoking pot while driving. Sometimes she would get pulled over for swerving but police would let her go when she didn't test positive for alcohol.

Whitney said part of the allure of marijuana for teens is that it's easy to hide.

"You can get high in less time than you can get drunk, and your parents won't detect it," she said.

Dr. Jeffrey Runge, the head of the National Highway Traffic Safety Administration, said states are training police to recognize the effects of various drugs, but said more training is needed.

Runge also encouraged states to test drivers for drugs after a crash so officials can understand the scope of drugged driving. Now, drivers rarely are tested for drug use, Runge said. One-quarter of the 3,657 drivers age 15 to 20 who were killed in accidents in 2003 had a blood-alcohol level of 0.08 or higher.

Runge said teens are at special risk because they are inexperienced drivers and they often have a dangerous combination of alcohol and drugs in their systems. He said teens must understand the dangers and designate a driver before they go out.

"Every driver has a personal responsibility not to get behind the wheel while impaired," Runge said. "A designated driver does not mean the least drunk or stoned person at a party."

In the television ad, which will run through the end of the year, a young female driver keeps having visions of an older man after she hits him in a crosswalk.

"You'll never forget the people you hurt when you were high," the narrator says.