Iowa Code § 321.455: Projecting loads on passenger vehicles.

No passenger-type vehicle shall be operated on any highway with any load carried thereon extending beyond the line of the fenders on the left side of such vehicle nor extending more than six inches beyond the line of the fenders on the right side thereof. Passengers shall not ride on any part of any vehicle unless it is expressly designed either for passenger use or designed for carrying livestock, merchandise, or freight.