MUTCD Section 2B.55 talks about ‘Photo Enforcement” signs clearly - A PHOTO ENFORCED word message sign may be mounted below a regulatory sign to advise road users that the regulation (like speed limit) is being enforced by photographic equipment. A TRAFFIC LAWS PHOTO ENFORCED sign may be installed at a jurisdictional boundary to advise road users that some of the traffic regulations within that jurisdiction are being enforced by photographic equipment.