Every trailer or semitrailer of a gross weight of 3,000 pounds or more, when operated on a highway, shall be equipped with a permanent metal frame attached to the underside of the rear of the trailer. The frame may not be any wider than the width of the trailer and shall be not more than 30 inches above the highway when moving and constructed of heavy gauge steel. The maximum transverse distance from the widest part of the vehicle at the rear to the frame may not exceed 18 inches. The frame shall be constructed so as not to interfere with lights or other warning devices. Any trailer or semitrailer so constructed and maintained that the body, chassis, or other parts of it afford the protection required by this section is in compliance with this section. This section does not apply to pole trailers or to vehicles where the installation of the required frame would prevent operation of the vehicle to secure its designed use.