No person may operate or cause to be operated any bus, truck, trailer, or semi-trailer upon any highway in this State unless the bus, truck, trailer, or semi-trailer is equipped with suitable metal protectors or substantial flexible flaps behind the rearmost wheels to prevent, as far as practicable, the wheels from throwing dirt, water, or any other materials on the windshields of vehicles following or approaching. The flaps shall be of a type and size as specified by the Department of Motor Vehicles. However, this section shall not apply to farm tractors or pole trailers. (1959, No. 105, §§ 1, 2, eff. Sept. 1, 1959; amended 1971, No. 228 (Adj. Sess.), § 32.)