Here's a map of where I've been. Biking on the roads is OK. The legend says "Off-Road Bicycling Prohibited."
The question is -- what's a road? Because there are more things that sure look like roads that are not on the map.
My assumption has been that these other things that look like roads are OK if there is no sign saying bikes are not allowed.
I been down a bunch of these, let's call them side roads. No problem. And I've run into two rangers (one on a bike!). Had a chat with the other one -- looking afterwards at exactly where we were, we weren't on a mapped road. And she was as friendly as could be.
What's the saying -- better to ask for forgiveness that permission? :-)