theappalachianonline.com says:
It shouldn’t be illegal for drivers to signal other drivers about a speed trap—it should be considered “freedom of speech.” It’s my right to communicate with other drivers, whether I am showing them how to count to one or if I am warning them about a speed trap. ...

[After being cited for flashing his headlights to warn others of a speed trap, Erich] Campbell “filed a class action suit which says Florida Statute 316.2397 – under which Campbell was cited – does not prohibit the flashing of headlights as a means of communications, nor does it in any way reference flashing headlights or the use of high beams,” according to wtsp.com.

Campbell’s lawsuit also claims that Florida Highway Patrol (FHP) “is well aware they are wrongfully applying the state law and they are doing it as a means of generating revenue. In 2005, a court order was even issued saying the state law doesn’t prohibit the flashing of vehicle headlights.”

Due to Campbell’s lawsuit, FHP and Orange County sheriff’s office have suspended writing tickets for this offense.

The problem is, the law is the same in North Carolina.