Speed cameras.

[suzublu]

When I was on the bike, I always gave the thumb down sign to oncoming riders/drivers to indicate a cash collection vehicle now, in the car, flash the lights and do the slow down wave, not only does it warn other motorists, it also slows them down, thus having served its purpose.
As an aside, did you know it's illegal to warn other drivers of a speed camera? Obstructing law enforcement, or something like that
Speed limits, for the obedience of fools and the guidance of wise men