Flashing Your Headlights

A common practice for as long as I've been driving, but is it ILLEGAL.

As long as I can remember there's always been the battle between Police, the State Patrol, and law enforcement vs. Drivers over the issue of flashing their headlights to warn other drivers about a possible speed trap.  I'm sure you've seen someone flash their lights at you and then not too far down the road--there is the cop car, ready to pull someone over for speeding.   People do this as a court