They may volunteer to be the one to get their friends home safely, but "designated drivers" often drink-even to a level that impairs them behind the wheel, according to a report in the July issue of the Journal of Studies on Alcohol and Drugs.

The study, of more than 1,000 bar patrons, found that approximately 40 percent of designated drivers had downed alcohol. What's more, most of those drinkers had blood alcohol levels that could impair their driving.

It's not clear why those designated drivers drank despite their role. Some of them might think that as long as they don't feel drunk they are all right to drive, says lead researcher Adam Barry, Ph.D., an assistant professor of health education and behavior at the University of Florida in Gainesville.

"People do try to use that as a measuring stick," he says. "But alcohol is insidious." That is, your driving skills are already impaired before you feel the "buzz" that tells you you've indulged too much.

"If you're going to be a designated driver, you should abstain from alcohol use completely," Barry says.

For the study, Barry's team went out into a college bar district six distinct nights (10:00 p.m.-2:30 a.m.) over three months, recruiting bar patrons as they exited drinking establishments. Ultimately, 1,071 people agreed to be interviewed and take alcohol breath tests-including 165 who said they were the designated driver.

About 40 percent of those drivers had been drinking. On breath tests, 17 percent had blood alcohol levels between .02 and .05 percent, while 18 percent were at .05 percent or higher.

Although people can legally drive with a blood alcohol level up to .08 percent, studies have found that alcohol begins to dull people's driving skills at a blood level of .02 percent. By .05 percent, the ability to drive safely is clearly impaired.

Of course, Barry notes, it's best for any driver-not just designated drivers-to refrain from drinking. But it may be particularly risky when a designated driver imbibes, because he or she will have a carload of drunken passengers.

"They may be loud, or start roughhousing. They're a distraction," Barry says. Couple all of that with the fact that most people drink at night, when any driver's vision is diminished, and you have a potential recipe for disaster, according to Barry.

A number of studies have found that designated-driver campaigns, although popular, have done little to actually prevent drunk driving. If trends like the one in this study continue, the researchers say, designated-driver campaigns will probably continue to disappoint.