About a century ago in the UK, the Automotive Association (AA) did a similar thing. Lots of towns were setting up speed traps to catch speeding motorists, with

About a century ago in the UK, the Automotive Association (AA) did a similar thing. Lots of towns were setting up speed traps to catch speeding motorists, with very low speed limits, so the AA paid uniformed boys on bicycles to find the speed traps and flag down motorists to warn them before they reached the speed trap. This practice was legally challenged as obstructing the police, so the AA inverted the scheme. They instead paid boys on bicycles to salute motorists by default, except when there was a speed trap. If you saw the uniformed AA boy standing by the side of the road and he wasn't saluting you, you knew there was something wrong ahead. The idea was that the law might be able to ban saluting to warn motorists of the police, but the law couldn't ban not saluting to warn motorists. The law couldn't compel a salute. Evidently this worked, because the AA kept up the practice for a few more decades, before eventually discontinuing the practice in the 60s.