If you live in a small town, you'll know what I'm talking about.
As you're driving, you see another vehicle and as you pass each other, you wave. (The left hand, two-finger wave, is my personal favorite).
I now live in a suburban community where, though not as frequently as in rural communities, you still see motorists wave or recognize other motorists. I don't wave at everyone I see, but several times a day I'll acknowledge others with wave or nod.