Or when you're tired, or when you haven't had anything to eat for too long and your blood sugar is low, or when you are engaged in a quarrel.   

And in fact it's always illegal to drive when you are unfit to drive, for any reason, it's just that with alcohol it's particularly easy to lay down a specific limit and enforce it. I think there'd be a good case for having checks on some of the other things too. "Don't Diet and Drive..."