Flashing your headlights to warn of cops

How come no one does this any more? Flash your high beams at oncoming cars to warn of a cop or speed trap that is.
Bikers are pretty good at warning each other but when I'm in the car almost no one warns ya. I try to warn people a km or two after but I feel like I'm one of the rare few. 5 or 10 years ago a lot more people did it but now no one does. Why?
Discuss...

Flashing your headlights to warn of cops