(1) A person may not ride and a person operating a motor vehicle may not knowingly permit a person to ride on any portion of a vehicle not designed or intended for the use of passengers.
(2) This provision does not apply to:
(a) a vehicle that is not being operated on a highway;
(b) an employee engaged in the necessary discharge of the employee's duty; or
(c) a person riding within or on a motor vehicle in a space intended for a load on the vehicle.