Where are bikes explicitly prohibited in MD?
The way I read the map, it is only prohibited on the Highlands Trail (Teal Trail) from Saffin Pond parking lot to the Headley Overlook. From the Weldon Road crossing just north of the lookout to Sparta Mountain Road it seems the Teal Trail shares the White Trail and the White trail is a well ridden pathway given all the nice single track off on various side trails.
The aforementioned section of the Highlands Trail is pretty rough anyway, especially those two creek crossings, as it is in other parks along its way to the NY line like in Newark Watershed, Norvin Green, and Long Pond Ironworks. But for the hardy, that would be quite a challenging cross state adventure.