Most of the side trails are currently off limits. If you take the open off shoots, they will meet up with the main fire road and take you to the end of the trail. The only one that does not is a dead end and the last time I was up there, someone had posted a sign stating that the trail was one.
I think the red line course indicates the more difficult side trails and a lot of those are not open anymore. You can see the ones that are still open because they just shoot off of the main dirt road.
Start from the 138, the area from the 15 to where the main trail head is located is just a flat road that's steep, i've seen locals driving their sedans down it to get to the 15 fwy.
In the map that fred1 posted, My cousin and I with our stock TJ and 4 door JK jeeps did all the red parts with only 1 part where both of our rear bumpers got hit underneath because of a sharp drop. This was before the heavy rains that we've had over the past couple of months so they might be harder now. They were a blast to do with great views though, if you have recovery equipment in case of unexpected mud then i highly suggest doing some of them.
if your going back the same way then you can check out the trail that leads to the 15 fwy just so you can say you've done it.