I don't know what it is about that little "courtesy wave" but I 
really like it. Driving is such an isolating activity and actual
communication between drivers is very rare and not at all 
positive when it does occur. The wave sometimes almost looks
and feels like a "hello" as well as a "feel free to cut in" or a
"thanks for letting me cut in." It might be the nicest inter-
action I'll have all day.