WASHINGTON (AP) -- An estimated 11 million Americans, including nearly one in five 21-year-olds, have driven while under the influence of illegal drugs, the government says.

The numbers announced Tuesday were especially high for college students. Eighteen percent of students surveyed said they drove while on drugs last year, compared with 14 percent of their peers who weren't in college.

John Walters, director of the White House Office of National Drug Control Policy, said the statistics show a failure to convince drivers that drugs impair driving as much as alcohol does. His office is kicking off an ad campaign to warn teens about driving while smoking marijuana.

"Marijuana is not the soft drug. Marijuana is not the casual rite of passage," Walters said at a news conference. "We have been sending the wrong message."

Walters said marijuana can affect concentration, perception, coordination and reaction time for up to 24 hours after smoking it.

Officials also cited a recent study that said 15 percent of high school seniors have driven under the influence of marijuana. That study, published in May, analyzed 2001 data collected as part of the University of Michigan's annual "Monitoring the Future" study, which questioned 44,000 students in 424 public and private schools in the country.

Nineteen-year-old Theodore Stevens of New Jersey told reporters that he believed smoking pot and driving wasn't dangerous despite getting into four accidents in three years. He says he's lucky none of those incidents caused serious injuries.

"Sometimes I believed it increased my driving performance," said Stevens, who has been in drug treatment for four months after being charged with possession of marijuana, cocaine and heroin. Stevens began smoking pot when he was 14.

The report, compiled by the U.S. Department of Health and Human Services, used 2002 data from the National Survey on Drug Use and Health. The survey questioned 68,000 people. Researchers then extrapolated the percentages to the population as a whole. A federal statistician said the margin of error was plus or minus 4.5 percentage points.

For 21-year-olds, the rate of those who reported driving under the influence of drugs was 18 percent, the highest of any age group. That dropped off to 14.5 percent for 22-year-olds. Unemployed adults age 26 to 49 also had a high frequency of driving while drugged -- 9.3 percent, compared with 5.1 percent for drivers employed full time.

Among racial or ethnic groups, American Indians reported the highest rate of driving while drugged, at 6.3 percent compared with 5 percent of whites, 4.5 percent of blacks, 3.7 percent of Hispanics, 3.1 percent of Pacific Islanders and 1.3 percent of Asians.

Dr. Jeffrey Runge, head of the National Highway Traffic Safety Administration, said there were approximately 38,000 crashes last year involving drivers impaired by marijuana. But Runge said he didn't know how many fatal accidents were caused by drugged drivers. State data collection is spotty, Runge said, and many drivers who are driving while drugged are also drinking.

"While we don't have fixed data, impairment is impairment," he said.

Copyright 2003 The Associated Press. All rights reserved.