Originally Posted by LexTalionis
This. I've ridden all the legal roadways in that area with a park ranger (for San Jose) who lives a few miles from the park, and there are no dirt roads legally passable by the public.
True that there are no *through* dirt roads open to motor vehicles, but the last mile or so of upper China Grade (up to Johansen Road at the Boy Scout camp sign) and White House Canyon/Creek (accessed from Hwy 1) are both dirt and open to the public (at least to the point where they are each posted closed).
Upper Gazos Creek (up to Sandy Point) is also fun (but it's mostly broken pavement) when it is seasonally open.
None of these are super exciting in themselves, but I like to head up them occasionally for some variety on a larger SC mtns ride.
Big Basin Brochure (map at bottom): http://www.parks.ca.gov/pages/540/fi...Layout2013.pdf