I've spent quite a bit of time asking forest service and BLM officers in the assorted offices I've been snagging maps at. In Utah, most of the BLM and Forest service managers have agreed on 50 inches as the limit for ATV trails, which is tighter than the 52 inches many of the areas were designated at a couple years ago. It's still difficult to find good maps for many areas as the land managers have just recently built their motor vehicle use plans. In the areas that are done and mapped out though, the trails tend to be open to all OHV, or restricted to 50 inch or less. As for state or county land, it still varies. Many of the gates however are still sized for 52 inch vehicles.
From what I've seen of the trails in Southern Utah, all ATV trails there are now designated 50 inch or less on the vehicle use plans, but most don't have gates, just signs that show atv, but restrict jeeps. They count on you to know what sort of vehicle can be driven on the tail.
