In the UK during the early era of cars, the Automobile Association used to send boys out with bicycles to warn drivers about speed traps. This was challenged in court, obstruction of justice or some such, so the AA simply inverted the scheme. The boys were now told to always salute cars to signal that everything was okay, but wouldn't salute if there was a speed trap ahead. It was reasoned that the law couldn't compel the boys to salute. Apparently they kept this up for a few decades, before eventually deciding that speed limits were generally reasonable.