The concept of a designated driver is a very, very simple one: This is the person who is designated to not drink and to drive his or her companions home at the end of the night.

Easy in concept but apparently a bit difficult in execution. About one-third of designated drivers have at least one drink while carrying the title, according to a new paper in the Journal of Studies on Alcohol and Drugs. Twenty percent, breath tests showed, had a blood alcohol level higher than 0.05, enough to impair their driving skills.

The design of the study was quite simple. Researchers stood outside of bars near a college in the Southeast on weekends:

Data collection transpired on Friday evenings before scheduled home Saturday football games, beginning at approximately 10:00 P.M. and ending by 2:30 A.M. Patrons exiting bars on public sidewalks were recruited to take part in a structured, 3–5 minute survey assessing alcohol-related behaviors and basic demographic information. Trained recruiters obtained verbal informed consent from each participant before data collection. At the conclusion of each interview, the participant’s BrAC was measured using a handheld alcohol breath testing instrument.

All told, the researchers collected data from over 1,000 people with an average age of 28. Fifteen percent of the sample identified themselves as a designated driver.

"It is noteworthy that nearly 40% of DDs did not refrain from alcohol use," the researchers write. "Of the 165 self-identified DDs, only 65% had a .00 g/210 L BrAC reading."

As the researchers note, this study has limitations: It was done at a set of college bars with a relatively homogeneous population. It doesn't speak to designated driving in other situations. Still, it does suggest that there's at least a decent-sized segment of the population that doesn't have a strict idea of what it means to be the designated driver.