I thought the supreme court already ruled that warning other motorist of a speed trap was covered under the freedom of speech.
You can't always trust Wikipedia, but...
In the United States, the legality of headlight flashing varies from state to state. Historically, law enforcement officers give citations for headlight flashing under three types of laws: (1) laws prohibiting a person from obstructing a police investigation, (2) laws prohibiting a person from having flashing lights on their vehicle, and (3) laws prohibiting shining a vehicle's high beams at oncoming traffic. The specific language of each law varies by state along with courts' holdings on whether their respective laws prohibit headlight flashing.
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