No commercial vehicle, as defined by state statute, or a vehicle displaying a merchant/business loading or customer pick-up sign issued by the Greater Frederick Development Corporation shall be permitted to be parked in any loading and unloading zone for the purpose of loading and unloading goods, wares and merchandise for a period exceeding twenty (20) minutes. (Code 1953, § 10.102; Ord. No. G-92-6, § 1, 3-5-92)