No operator of a truck, trailer or other vehicle equipped with a tail gate shall lower or open the tail gate thereon, or suffer or permit such tail gate to be lowered or opened, except during the time the vehicle is being loaded or unloaded, and except during the time the load on the vehicle necessitates a lowered or opened tail gate as a support for the load. It shall be the duty of the operator of any such vehicle to see that the tail gate on such vehicle is kept closed or raised, except during the times specified herein.