BACtrack S80 Pro Breathalyzer Portable Breath Alcohol Tester. -- $120 on Amazon
It's not going to solve the problem for people with terrible judgement, but it can help. I have one I carry in my laptop bag, which I have to have with me just about at all times for work. If I'm at a party and somebody shouldn't be driving I'll offer it up. And really it's hard for them to say no. Really? No? You're "good"? Come on man, it's fun. And it is fun (in a nerdy way). So far it's saved one friend, you know sort has had a little too much, but boy.. had a lot to eat and it's been hours since his last beer, but it's pretty late, but he did just drink a few coffees. one of those situations where you know he shouldn't drive, and he sort of knows, but his wife is going to kill him if he stays over....
So he blew over and that was it. Right there, over the limit. No question, no "I'm OK, it's not far" or "I'm just tired I'll be fine." Nope, we all just saw that, you are over, nobody is letting you leave. This is the smart thing to do.
Anyways sort of a tangent but this thing is money well spent. Hope it helps somebody else.