Something I was told a long while back was that the proper warning to a fellow rider when seeing them on the road was to tap the top of your helmet to warn them of cops coming up.
I've always done that when I know a rider is about to come up on a cop or about to encounter a police action (like an accident scene) - but to be honest I haven't a clue if anyone even knows what the H I'm doing and I might as well stop doing that.
Are there other hand signals to pass along to a fellow rider to help them out - something like "rest stop" or "food". Seems like when you're traveling the back roads it's nice to have someone pass along some information - like "Don't worry there's gas up ahead" or "watch out, dangerous road".
Harley riders have their own greetings that border on arcane symbolism - but that's not what I'm talking about here. Given the ubiquity of GPS and cell phones (I don't use GPS when riding) maybe this is a lost art that no longer serves a purpose?