Nothing contained in this article shall be construed to prohibit commercial vehicles from using on-street public parking spaces as provided in this article, when such commercial vehicles are actually engaged in the loading or unloading of merchandise to commercial business houses in front of which parking meters are located at such times as such parking spaces are not otherwise lawfully occupied; provided, that such commercial vehicles shall not use said parking spaces for any longer period of time than is actually necessary for such loading or unloading; provided further, that in such cases, such commercial vehicles shall not be required to make any deposit of coins in such parking meters. (1978 L.M.C., ch. 7, § 2.)