I used to think the same - I've lived in plenty of places with no public transportation. No choice, I'd say, bemoaning the lack of decent public transport.

But then I was in a similar situation off in a small town in Norway. I was at a 80th birthday party in a small town. Most of the attendees were drunk. Everyone already knew someone to take them home, and organized themselves accordingly, some waiting a bit for the sober driver to return empty. A few found someone at the party (my spouse)- and then proceeded to drink. It was nearly surreal to be in the situation.

The truth is that in the states, it is OK to say, "I've only had X beers, so I'm ok to drive." IT is socailly acceptable behavior. And no one really keeps track most times, so if they seem sober enough to us or we know better, we'll let them go. The acceptable BAC level is much lower here - one beer can send you over the limit - and penalties are harsher, so that could be a major factor as well.