It isn't quite so simple. Much depends on the duration of time over which the alcohol is consumed, whether it is with food or not and how used to alcohol a person is. The quantity of alcohol in the blood is not a clear indication of how "drunk" a person is, but it is an easily carried out procedure. Some form of physical test would be better, once upon a time suspected drivers were asked to walk in a straight line.
To person who rarely drinks, one drink can make them dizzy, but it would have little or no real effect on a regular drinker.
I recently had 2 litres of 5% strength beer and a shot of ouzo, but over a 2 1/2 hour period with a lot of Greek food. On the way home I was stopped by the police in a random check, breathalysed and was clear.