the amount of people who have said dont drink and drive, etc etc yeah of course you are right in an ideal world, but guess what we dont live in an ideal world,
the breathaliser is just an indication of how many units you have. it dont mean that you are gonna be pissed and all over the road, a lot of people can have 5 beers and drive safely home. its a choice you have to make, if its more convinient for you.