On the tandem, part of the stoker's 'duties' is to wave at all cyclist going in any direction.
On single, I wave, nod, or say 'hi.'
Also we acknowledge/thank motorists that give us a break . . . and always wave at the police!
Hey, takes no effort to be nice/polite/friendly . . .

Pedal on TWOgether!
Rudy and Kay/zonatandem