A little different in my neck of the woods. It's a rural area, but the road going by our property has a 55mph speed limit. If I'm out near enough to the road and recognize the car (by sight or if they honk) I will surely wave. If I am pre-occupied they'll have to honk to get a reaction. If I stood there to watch and wave at every car that went by, two things would happen, nothing would get done and my arm would likely fall off.
I gotta say though, the people around here are real friendly. Even if I'm not out there, a lot of them will leave something behind on their way by to make sure I knew they were there. It's not usually anything big, sometimes just an empty beer bottle or maybe a McDonnie's bag, but it seems that quite a few of them are kind enough to take the time out of their busy day, and hey, it's the thought that counts.