Impairment in judgment can begin at much lower levels than the 0.05 g/100ml blood alcohol concentration (BAC) level that makes it illegal to drive in South Africa.
Yet people still think that's the magic bullet, and any amount consumed below that means it's safe to drive, says Maurice Dennis, director of the Centre for Alcohol and Drug Education Studies at Texas A&M University.
Dennis recently completed an as-yet unpublished study that suggests differently.
Losing control of the vehicle most common
Dennis took 19 people of varying ethnicity and age and trained them in driving behaviour. "Not everyone can drive equally well to start with," he explains.
Six of them didn't drink any alcohol. The rest were offered any type and amount of alcohol they wanted. When it was thought that their BAC was about 0.04 percent, the volunteers had their blood and breath analysed for alcohol levels, and they were then asked to drive through a closed course.
The course included six different exercises of varying difficulty. In three of the simpler tasks, they were asked to perform exercises such as driving between cones. In the more difficult driving tests, they had to do things like maintain control of the car as they came out of a skid. "That's where they had greater losses," Dennis says.
Evaluators watched the drivers and rated them, but did not know who had consumed alcohol. When the subjects completed the first driving test, they drank more until their BAC was about 0.07 and then were re-tested. The last driving test was done when their BAC was 0.10.
"We saw losses [in skill] even at .04," Dennis says. "These increased at higher levels."
Poor decisions in an emergency
On average, a small woman will have a BAC of 0.04 after just one or one and a half beers, he says. For a 68kg man, it's closer to two beers.
Usually, imbibing a bit and driving are not a problem, Dennis notes. It's when "something out of the ordinary happens," he says. "You've taken away the edge."
Dennis cites the example of driving sober but occasionally running a red light. Usually there may be little traffic, and your indiscretion is not a problem. However, as you continue to run red lights, your chances of being involved in an accident increase dramatically, he notes.
The same holds true with driving after drinking a bit. Under normal circumstances, all is fine. But one day, you may have had a few, get behind the wheel, and be behind someone who suddenly slams on the brakes, leaving you to decide to brake or swerve into another lane where there may be a pedestrian or other car.
In Dennis' experiment, when a driving emergency arose, those who had just a beer or two tended to make poor decisions, he says.
Get someone else to drive
Some impairment in judgment definitely exists by a BAC of 0.04, agrees James Fell, who is on the national board of directors of Mothers Against Drunk Driving.
"Plenty of people are involved in crashes at lower levels. The probability of being involved in a crash increases at every level [of alcohol consumption]," he says.
"We're not prohibitionists," Fell asserts. But "if you're going to drink, make sure someone else drives for you.