1. What they do is their own business.2. There is no way in hell I'd ever get in a car with someone who is drinking.
I think most people who know me know a friend was killed by a drunk driver when we were in Junior High. Yes, I STILL refer to this each and every time it has come up ergo I don't get in the car. I've paid for taxis when I've had a few drinkies, I've driven other people's cars, booked hotel room nearby... there are many solutions.
I'm finding this really hard to figure out. So, you are saying:
1. If people I know drink and drive it is not my business.
2. It is my business to find alternative transport for myself (because I once had a friend who was killed by a drunk driver and I am therefore worthy of sympathy) if the people I have been drinking with are liable to get on the road and kill themselves and others -- because I don't want to be killed myself.
3. It is not my business at any time, even though a friend of mine was killed by a drunk driver, to intervene with a person who drinks and drives. Because it is their business who they are likely to kill, and I am innocent because I took a cab to preserve my own life, in full knowledge that the driver was impaired.
Perhaps you didn't mean it, but that logic sounds rather odd to me.
I've never been in this situation before so obviously it was not something we had ever discussed and he didn't realize I felt so strongly about it. 
I had never driven a van before and this thing was huge!! Luckily there was no interstate driving involved!
), then maybe the rest of us can help and offer advice as to whether this is truly a problem or not. If you just get snippy though, well then no one can help anything.