One blink was 'Hi.' Two was Cop ahead; three was danger ahead (road out, bridge out, icy bridge). A single flash when being passed was "I got you, okay to pull back in." Turning off the lights briefly was "thanks."

There was a time when a Cop witnessing a double flash to oncoming traffic would pull you over and harass you just for grins. Haven't seen that happen in the least 30-40 years.

Back to the thread -- you can bet there were and are literally hundreds of signals -- to include what the kids are doing and how they're playing. Also verbal cues that bystanders or even many Terps may pick up but not reveal.