He can say that cops are ahead. He can’t say “slow down.”
I believe he altered his sign and is back on his mission. No Constitutional crisis.
Actually, yes, he can.
11.50.560 - Forbidden devices. A. No person shall erect or maintain at or near a street or alley any structure, sign, light or device that is:
1. Visible from a street or alley and simulating any directional, warning, or regulatory sign or likely to be mistaken for such a sign or bearing any such words as "danger," "stop," "slow," "turn," "impound," or similar words, figures, or directions likely to be construed as giving warning to or regulating traffic;
Neither of those two items I put in bold apply here. His sign is not simulated, his sign is not likely to be mistaken for an actual traffic sign. It's a lid.