Is it ilegal to use my digital bumper sticker and digital message board (usually says Uber) to warn people of police presence?

I saw an officer hidden between old building and deserted car was n just barely noticed in time slow down so I use my digital message board I usually use for Uber sign and put up police ahead but last night I I did it and was pulled over and asked to take it off I asked if it's illegal and they would not answer... show more I saw an officer hidden between old building and deserted car was n just barely noticed in time slow down so I use my digital message board I usually use for Uber sign and put up police ahead but last night I I did it and was pulled over and asked to take it off I asked if it's illegal and they would not answer just said take it off so Informed him he was being recorded (inside rearview mirror has camera built in) and the officers then asked for liscense registration my insurance card 15 minutes later said I'm free to leave. So is it legal for them to pull me over and ask me to take something off my vehicle like that? Is there ANY law that says I can't warn other drivers about police presence? Because to me it seems like they just pulled me over because they were offended and tried to MAKE me take it off until they thought they were being recorded.
11 answers 11