It is unlawful for any common motor carrier operating in the city to allow motor vehicles to stand on any portion of Main Street, or on any street within one block thereof, for a longer period of time than is necessary to load and unload such motor vehicle. Any common motor carrier operating any motor vehicle in the city shall maintain parking stands for the motor vehicle off the street and on private property, and the motor vehicle shall be parked in the parking stands when not in operation. (Prior code § 18-15).