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.