I feel like Waze has killed the high beams flashing warning, although I miss it. It sounds crazy as a say it out loud, but the signals between drivers always gave me warm feelings of some kind of community with other drivers. Now everyone on the road seems much more internally absorbed or focused. Cars are quieter, podcasts instead of radio, etc. While they were never very common, CB radios were also less rare in cars (we always had one for road trips and both my grandparents had one in on all the time in their cars).

what's waze?

A navigation app owned by Google, which is popular largely for its police and hazard notification system. Apple has recently copied this feature in Maps, and you can use Siri to report hazards/etc. (which I believe cannot be done in Waze, at least on iOS).

Some app that a small minority of people use which includes warnings about traffic, hazards, speed traps, etc.

There's two common motorcyclist signals for police: Either tapping the top of the helmet, or pointing up at the sky while moving your arm in a circle (to mime spinning police lights).

Other motorcyclist signs are sticking the left leg out to say thanks, holding your hand out and rapidly opening/closing it to say "you've left your indicator on", and pointing at dangerous spots on the ground like potholes, gravel, or other unseen hazards.

Flapping your arm like a child pretending to be a bird is another I've seen warning of a speed trap (it's actually the hand signal for "I'm slowing down" back in the days that was relevant).