No person shall stop, stand or park a vehicle between the hours of 7:00 o’clock A.M. and 6:00 P.M. on any day except Sunday in a commercial loading zone indicated by yellow curb except that commercial vehicles may stop or stand for the purpose of loading or unloading freight for the time necessary therefor, but not to exceed 20 minutes. Passenger vehicles may stop or stand for the purpose of loading or unloading passengers and their personal baggage and then only for the time necessary therefor, but in no event for more than three minutes.