Drunken driving is down. But what about drug use by drivers?

Americans seem to have finally gotten the message: It's not OK to drive while drunk. But driving on drugs appears to be another matter and a significant problem.

The National Highway Traffic Safety Agency (NHTSA) on Monday released the results of its 2007 Roadside Survey of Alcohol and Drug Use. It found that the share of Americans driving while intoxicated on weekend nights is at a low of 2.2 percent. That's down 71 percent from the mid-1970s, when the survey was first conducted. Back then, 7.5 percent of the drivers surveyed had blood alcohol-content levels higher than the legal limit.

That's the good news. But for the first time, the survey also looked at drug use late on weekend nights. It found that 16.3 percent of drivers had traces of drugs in their system, including marijuana, cocaine, and various stimulants, sedatives, antidepressants, and narcotics. But the survey's authors cautioned that it was difficult to tell from the findings whether the drivers were impaired, because traces of some drugs like marijuana can be found weeks after a chronic user has stopped taking the drug.

Still, analysts say, the findings indicate there is work to be done on both fronts.

"Even though the alcohol numbers are the good news, we're still seeing over 10,000 fatalities a year involving a legally impaired driver or motorcyclist. So we still have work to do," says Rae Tyson, an NHTSA spokesman. "On drugs, the numbers are fairly high and representative of drug use in society, but what it says about the impairment of the person behind the wheel remains to be seen. "

For years, law-enforcement officials have encountered drivers who seemed to be impaired but did not test positive for alcohol. That prompted the NHTSA to start research on how drugs can affect an individual's ability to drive safely. One issue still to be resolved is how to test for, and find a safe driving threshold for, drugs that can stay in a person's system for days or even weeks.

"We know what that threshold is for alcohol, and states have universally set 0.08 blood alcohol content as the limit," Mr. Tyson says. "We still have a lot of research to do and a lot to learn in terms of what impact drugs have on your system and at what point a driver is too impaired by drugs to be behind the wheel."

For groups like Mothers Against Drunk Driving (MADD), which has been working to get intoxicated drivers off the roads and to educate young people since 1980, the survey results on drunken driving are encouraging.

"Most of the credit needs to go to the law-enforcement community for stepping up and ... doing sobriety checkpoints and getting the drunks off the roads," says J.T. Griffin, vice president for public policy for MADD, which is based in the Dallas area. "But we still have a long ways to go."

Mr. Griffin says that MADD does have "major concerns" about the number of people driving under the influence of drugs, but the group will keep its focus on drunken driving.

"For a lot of these folks, the drugs and alcohol use go together. So if you can still focus on the alcohol piece, in a lot of cases you'll catch the drug-impaired drivers as well," says Griffin.