The road down from the highway through the underpass is certainly
passable...with 4 wheel drive. I have no problems getting down 
there in my 4Runner.

The entrance is hard to spot from 50 mph on the highway, but if
you're approaching the Loop from Portola, once you go past Spring
Garden there's a big bend in the highway to the right followed by
a sweeping curve to the left. Look carefully and you'll see the
loop track just off the highway (but you can't see the tunnel).
Just beyond this is the entrance, a narrow break in the hillside
with a dirt road leading south off the highway. It's directly
across from a yellow highway sign (Curve ahead or winding road-
I don't remember which, I just use the location as a marker).
If you cross a highway bridge with concrete railings, you've
missed the turnout and will have to go back. I know it sounds
hard to find, but once you do find it, it'll be easy to find
again!