The police chief or administrator of the traffic bureau is authorized to issue special written permits with the approval of the director, division of traffic engineering to permit the backing of a vehicle to the curb or over the sidewalk for the purpose of loading or unloading merchandise or materials subject to the terms and conditions of such permit. Such permits may be issued either to the owner or lessee of real property or to the owner of the vehicle and shall grant to such person the privilege as therein stated and authorized herein, and it shall be unlawful for any permittee or other person to violate any of the special terms or conditions of such a permit.