The only time I, as a jaded, cynical, suspicious Londoner, ever make any form of gesture of appreciation towards any other road user is when I am stationary in my car at a traffic light or other junction and I see a particularly nice motorbike.

I then give them the thumbs up sign while making an approving nod of the head gesture. They always acknowledge in a kind of 4 wheels & 2 wheels mutual respect kind of way.