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Will you be safe letting your designated driver take you home? New research reveals that you won't be sure 40 percent of the time.

While people may sometimes volunteer to get their friends home safely, a new study reveals that two out of five designated drivers drink before getting behind the wheel. What's more, researchers found that half of these drivers have a blood alcohol content (BAC) of 0.05 percent, which is enough to affect driving ability while still being under the legal limit.

The study, conducted in a Florida college town, involved more than 1,071 participants leaving bars in Gainesville, Florida on a series of Friday nights over a three-month period.

The findings revealed that 165 of the subjects were deemed "designated drivers" by their friends.  However, approximately 40 percent of these designated drivers had had something to drink.  More specifically, 17 percent of them had blood alcohol content between 0.02 percent and 0.049 percent, while 18 percent had at a BAC of 0.05 percent or more.

The average age of the people in the study was 28. Participants in the study weren't particularly diverse and were mostly white male college students.

Researchers note that while people can legally drive with a blood alcohol level of up to .08 percent, previous research has found that alcohol begins to affect people's driving abilities at a blood level of .02 percent, and by .05 percent, the ability to drive safely is clearly impaired.

Lead researcher Adam Barry, Ph.D., an assistant professor of health education and behavior at the University of Florida in Gainesville said it's best for any driver, not just designated drivers, to refrain from drinking.  However, he stressed that it may be especially risky when a designated driver drinks because he or she will be in the car with other drunken passengers.

"They may be loud, or start roughhousing. They're a distraction," Barry said in a news release.  Furthermore, most people drink at night, when any driver's vision is diminished. He said all those factors added together equals a potential recipe for disaster.

While it not clear why the designated drivers in the study drank.  Researchers say some of them might think that as long as they don't feel drunk they are all right to drive.

"People do try to use that as a measuring stick," Barry said. "But alcohol is insidious."

Barry noted that popular designated-driver campaigns have done little to actually prevent drunk driving. 

"When you look at evaluations of designated driver campaigns, they're really ineffective," Barry told The New York Times.

"Often people choose designated drivers because they're the ones who've drunk the least. The most practical recommendation is that if you drive, you shouldn't drink at all," he added.

The findings are published in the Journal of Studies on Alcohol and Drugs.