And one in 12 drivers think it would take four units or more to affect their driving, according to road safety charity Brake and Green Flag Motoring Assistance, which conducted the survey.
Brake chief executive Mary Williams says: "The findings show alarming confusion among drivers about how much they can drink before getting behind the wheel. The message that if you’re driving, don’t drink a drop is not getting through."
The results of the survey coincide with the launch of alcoholic lock trials by the Department for Transport.
The Government department is fitting ‘alcohol ignition locks’ to the cars of convicted drink-drivers as part of a research project in Birmingham and Bristol. The locks prevent a car starting if a driver registers as over the limit after blowing into the device.
If the pilot succeeds, new legislation could be introduced allowing courts to use alcohol locks as part of drink-drive rehabilitation programmes.
Road Safety Minister David Jamieson, says: "Over 3,000 people are killed and seriously injured by drink-drivers every year and we know that 20% of perpetrators are repeat offenders. Drivers who have been banned in the past could have the alcohol lock device fitted to stop them driving even after half a pint or a small glass of wine.
"Alcohol lock programmes in other countries have worked well with repeat drink-drivers and it is vital that we look at how the technology could be received here too. There are a lot of issues to be explored but I would like to see it legislated for as soon as possible."
Williams welcomed the idea of alcohol locks but believed greater enforcement was also needed. She says: "Introducing technology such as alco-locks is good news for road safety, but there also needs be more police on the beat to stop the year-on-year increase in drink-drive casualties."
What is the legal limit?
The legal limit for alcohol in breath is 35micrograms in 100ml of breath. In blood it is 80 milligrams per 100ml and in urine it is 107 milligrams per 100ml.
What constitutes one unit of alcohol