How do people drink at restaurants and then drive safely?

I’m freshly 21, I know most of the basics about types of drinks and what I enjoy, but I’m still about to be asking a noob question—how on earth do people go drinking at restaurants and then drive themselves home safely? I know that alcohol affects people differently based on biological factors, what I mean is like… how does the individual know when they’re toeing the line for risking a potential DUI if they get pulled over? How do you know, in a restaurant without a breathalyzer, when you’ve had enough and should stop while still being under the legal limit? Or is everyone out here just eyeballing it?