It shall be unlawful for any person to place, maintain or display any device, other than an official warning or direction sign or signal authorized by Statute or ordinance, upon or in view of any street, if such device purports to be, or is in imitation of any official warning or direction sign or signal, or directs or purports to direct traffic. Any such unauthorized device is hereby declared to be a nuisance, and may be removed by a policeman.