Wis. Stat. § 346.92: Illegal riding.

(1) No person shall drive a vehicle when any person other than an employee engaged in the necessary discharge of the employee’s duty is upon any portion thereof not designed or intended for the use of passengers.

(2) No person other than an employee engaged in the necessary discharge of the employee’s duty shall ride upon any portion of a vehicle not designed or intended for the use of passengers.

(3) This section does not apply to persons riding within truck bodies in spaces intended for merchandise or to the operator of any such vehicle.