There's an interesting case going on in Texas where a man who was holding up a sign that said "Police Ahead" in the median of a road was arrested. He was even handcuffed and take in to custody.

According to ABC News, since Texas doesn't have a law prohibiting warning drivers about the police, authorities charged him under an ordinance that limits advertising signs to private property; the man, Ron Martin, was in the median of the roadway. He maintained in court that he was not advertising anything. He claims he is an ally of the police and that they have the same goal.

I tend to believe him; if the purpose of a speed trap is to get people to slow down, then Martin is helping in that endeavor. If the goal is to write as many tickets as possible so as to raise as much revenue as you can, then no, he is an adversary of the police. It is mentioned in the article that New Jersey is one of the states where motorists are legally allowed to flash their high beams at other drivers as a form of communication; many states don't even allow that.