No person shall drive any vehicle of any kind on any public street which has placed thereon or affixed thereto any sign, plate or other fixed objects which contains the words “Police,” “Police Department,” “L.A. Police Department” or “Department of Police,” or letters “L.A.P.D. Press,” or any other words or letters or group of words or letters of like or similar character or nature, unless such vehicle is owned and operated by a regularly organized department of police of a body politic, or such vehicle which though privately owned, is used by a duly authorized officer or employee of a department of police of a body politic in the performance of his duties.