Other than the brief pre-Christmas drink-driving campaign (which seems to be dying) there's really no enforcement – things are getting like the US, where it still traditional to drive home after a night out if 'you feel ok'*. Drivers don't believe they're going to crash until they do, and they're only likely to be breathalyzed after the fact, so no real deterrent effect.
*I confess, I used to, nothing extreme but it was usual to drive home after a couple of drinks. The police couldn't breathalyze you without due cause, so you had to fail a field sobriety test first.