[thatBilly]

The British Automobile Association (AA) used to have a network of operatives on bikes (cycle scouts) who would salute members displaying the AA badge if they were approaching a police speed check.

This warning activity was tested in court and found to be illegal, as interference with the police undertaking their duties. Their response to the judgement was to switch the warning method to NOT saluting members if they're approaching a speed trap because apparently they couldn't be found culpable for inaction. So they would only salute members if the coast was clear. A bit like a warrant canary.

[yetihehe]

In Poland people used to blink their high beams when there was speed check ahead, it's sometimes still practiced (illegal then and now, but not because you warn of police, it's classified as "misuse of lights").