It is unlawful to stand or park any vehicle, other than a bus in an officially designated bus zone, or to stand or park any vehicle other than a taxicab in an officially designated taxicab zone; except, that the operator of any passenger vehicle may temporarily stop in any such zone for the purpose of and while actually engaged in the loading or unloading of passengers, at such a time and in such a manner that does not interfere with the usage of a zone by busses or taxis.