"Flashing your lights to warn oncoming drivers of a police check, speed camera and so on (seems to be international)"
is a civil practice or a criminal conspiracy, here's another data point; Time, March 10 1958, Miscellany:
PICKET In Ardmore, Okla., State trooper Paul Clark was waved down by a man at the side of the road who got himself arrested when he stuck his head in the window, warned "Hey, the highway cops are working a radar trap over the hill."
The practice was obviously at that time viewed as both rare and reprehensible.
Mind you, different times, different mores; also in that Miscellany
Man's Woman In Springfield, Ohio, Marjorie June Flax drew many admiring male glances in a packed coartroom when she dropped assault-and-battery charges against her husband, said: "It was my fault; if I'd kept my big mouth shut, it wouldn't have happened."
It was probably reading that that pushed Betty Friedan over the top.