this is one of those tough issues. imo, it's too many variables and puts too much of an onus on bartenders. especially when it's not just the bartenders serving drinks but also waitresses.

i live near a college town and can still remember drinking in my college days. you go to a bar near campus, one out of five people maybe drove there. some came in groups (and thus a driver), some walked, others used their bus passes. but a bartender and a waitress does not know this. they'd have to limit everyone and thus cut out a good bit of income. not to mention many times you drank BEFORE you went out.

and there's the fact that you may be planning all along to take a taxi home. but your server isn't going to know any of that.

then, you have someone who is well past the stage of should be driving a car but not be visibly intoxicated. especially if they drank somewhere else first. by the time someone is visibly drunk, it's too late, unless you're going to force coffee down their throats for the next several hours. you cut them off, so what? they are still drunk.

my brother is a bartender and he does deal with this some. usually it's by keeping track of how many actual liquor drinks he served someone per hour over time. but, again this can be completely out the window if the person came in intoxicated or went out to their car and smoked a joint before taking off (this actually happened, he totalled several parked cars).

but you try tracking that on a busy night when you're serving dozens of people coming and going, buying drinks for their friends and not themselves...

you really want to stop drinking and driving, you put the breathanalyzer in the car. over the limit, car doesnt start.