Bus stands, taxicab stands and stands for other passenger common-carrier motor vehicles, designated by appropriate signs, shall be as specified in appendix I, section 127, hereby made a part hereof. No person shall stop, stand or park a vehicle other than a bus in a bus slop, or other than a taxicab in a taxicab stand when any stop has been officially designated and appropriately signed, except that the driver of a passenger vehicle may temporarily stop therein for the purpose of and while actually engaged in loading or unloading passengers when stopping does not interfere with any bus, or taxicab waiting to enter or about to enter the zone.