Ive got news for you, every year the police get more and more agressive about stopping out of towners on the back roads. Depending on when you head in, you may be turned around and directed back to one of the main entrances, if that happens, you will be in line a LONG time!!
This is true as of last year when I got there Thursday afternoon. I walked in using the route you mentioned. There was a road block up ahead with a cop there. He didn't care about me walking by, but he wasn't letting vehicles through.