Way early season this year I was hitting gold creek for a break-in trail ride and the local sheriff pulled up behind me as we were unloading at the first switch back up from the Sno-park parking lot. The sheriff said if the berm is not in place and the sign "no wheeled vehicles" is not up then the roads are open to wheeled vehicles and ONLY wheeled vehicles. The reason is for safety. They don't wants cars and sleds operating on the same roads at the same time. He let us ride anyway even though there were 2 trucks that had gone past us and up further while we unloaded. His advice was be careful when you come around the corners.
I'm not sure if this is true to all sno-parks or just the main ones or maybe just the one he happen to be at. Anyway, take it with a grain of salt and do what you want. Just know that if the signs and the berms are gone, there is a possibility for a ticket and the occasional encounter with a wheeled vehicle. HAPPY SPRING RIDING!!!!!