The load upon any motor vehicle alone or an independent load only upon a trailer or semitrailer shall not extend to the rear beyond the last point of support for a greater distance than that equal to two-thirds of the length of the wheelbase of the vehicle carrying such load, except that the wheelbase of a semitrailer shall be considered as the distance between the rearmost axle of the towing vehicle and the rearmost axle of the semitrailer.