The flag rules from Silver Lake are below. Note: they are changing their entry / permit process this year to try to minimize the wait times. Might want to check it out prior to arriving. Think the big change is, permits are no longer being sold at the dune entrance but the visitor center.
New process is here :
DNR - Silver Lake State Park ORV Area Information
(7) Operate a vehicle requiring seat belts without a warning flag mounted and extending over the front-most portion of the vehicle. Vehicles not requiring seat belts shall mount the warning flag on the rear of the vehicle. Vehicle operators shall comply with all of the following requirements:
(a) The warning flag shall be mounted on a staff that is securely bolted, welded, or secured to the vehicle. The warning flag shall maintain a minimum 10-foot height at standstill and a minimum 8-foot height under motion.
(b) The warning flag shall be mounted on the top-most end of the staff. The flag shall be rectangular, international orange in color, solid material, with a minimum length on the supporting side of five inches and a minimum length on the base side of 10 inches.