But a new study, published in the July issue of the Journal of Studies on Alcohol and Drugs, finds that more than one in three self-identified designated drivers still drink.
Researchers find that, while designated drivers can legally get behind the wheel so long as their blood alcohol concentration is below the legal limit of .08 percent, driving impairment actually begins as low as .02 percent.
The team from the University of Florida surveyed 1,000 bar patrons over three months in Gainesville, Fla., and asked each participant if they were the designated driver and administered a breathalyzer test.
Of the 165 self-identified designated drivers, 35 percent had at least one drink, about half of whom registered a BAC between .02 and .05 percent.
The other half blew above a .05 percent.
"Considering the low BAC levels at which driving-related abilities are negatively affected, these findings identify the need for consensus across researcher, layperson and communication campaigns that a designated driver must be someone who abstains from drinking entirely," the study concluded.
"This is especially important considering that alcohol-related driver impairment, such as divided attention, is further exacerbated by the unsafe actions of drunken passengers," it said.
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