The problem becomes how long after that single drink? If I have a couple drinks over the course of an hour dinner, then go to the movies for 2.5 hours, am I good to drive home? How about if I meet a friend for Happy Hour after work, have a beer, and shoot the shit for an hour?

For me I'd be fine in both of those situations (by personal experience, and by weight / gender BAC estimates). The problem is many people overestimate their abilities while buzzed and/or underestimate how drunk they might be. People also tend to underestimate how much they've had to drink. Most BAC estimators use 1 beer as 12 fl oz of light beer, 1 glass of wine as 5 oz of regular wine, and 1 shot as 1.5 oz of 80 poof alcohol. A pint glass out at a bar has 33% more volume, and often stronger beer. Most mixed drinks contain at least 2 oz, and most people pour a bottle of wine 4 times, not 5.

I wish I had a good answer too, eg: "just wait until tomorrow morning". I've woken up a couple times from a night of heavy drinking still drunk. Really the best thing is to know your body, and know how much you're really drinking, over how long.