The park is shared among ATV's, dirt bikes, and trucks, so everyone has to have a flag for better visibility. Not sure how much it helps for the trucks, but it definitely makes the ATV's more visible.
I went wheeling in Pennsylvania at Paragon Adventure Park yesterday. Although it's not quite the same as adventuring out into the great unknown like in Arizona, after 7 hours of wheeling I'm sure I didn't cover even 10% of the park.
Met some folks from Gotham City Cruisers and had a great time...