When my dad bought his BMW 2002 in the early seventies there were around three of them on the road in the US, so we would wave at eachother when we passed one. Once they became more commonplace ( and BMW started appealing to yuppy poseurs ) , we stopped.

When I rode a motorcycle it was common practice to acknowledge eachother, although there was some protocol involved. You could wave at a similar styled bike ( i.e. standard, cruiser, rice rocket ) as yours. Once I made the mistake of giving the mini-wave to a scooter. He didn't know what the fuck I was doing and boy was MY face red!