Flashing the high beams to signal a speed trap or road hazard was common practice in up state NY in the 60's when I was growing up there. Can't say about the 70's or later - I wasn't there then………
Used to be common in Texas. Not so much any more. Kind of like almost every driver on a two lane in Texas used to give each oncoming driver a single finger salute. No, it wasn't the middle finger. That isn't seen much these days, either.
I seem to recall that flashing lights to warn of a speed trap was ruled as free speech.
Could be wrong though.
If you've ever lost a family member to a speeder then you might want them caught asap.
This news article is from 2014…
Federal judge rules drivers allowed to flash headlights to warn of speed traps | Fox News