In rural areas only have I ever seen anyone wave at anyone else. In cities of any size (25K pop. or more) look out and drive defensively.
I live in Georgia, in a moderately sized city, with somewhat more than the aforementioned 25K. There is a non-incorporated area in an adjacent county that's populated by mostly upper-middle-class, well-to-do, have more money than they know what to do with soccer moms, inconsiderate teens, and self-important banker-type dads.
These people literally and honestly believe to their core that when they're on the road, neither the rules apply to them, nor is anyone else allowed to interfere with their driving because they belong to the right country clubs, they go to the right churches, contribute to the right political candidates, and have Money. (see what I did there?)
Around where I live, courteous driving will get you run over. Better keep your head on a swivel, have an escape-out, and don't look another driver in the eye.Rural areas are much, much better. Everyone there knows each other...
I can't imagine what driving in a city of more than 60K people is like. And I don't want to know, either.
I am seriously thinking about getting a go-pro or other type camera, with a mic pick-up, and recording all of the incidents I see and have happen around me. You guys wouldn't believe it. And the worst part is, I never noticed it until I started riding on two wheels.