A person commits an offense if he stops, stands, or parks a commercial vehicle for any purpose other than for the expeditious unloading or loading of materials in any place marked as a loading zone during hours when the provisions applicable to such zones are in effect. In no case shall the stopping for loading and unloading of materials exceed 30 minutes, except on written permission from the chief of police or the director and then only for a period necessary to complete one undertaking. (Ord. Nos. 14584; 17413; 20269; 21194)