Park here and find the trail approaching North Peak and Bridge Mountain just up the hill.
Driving directions from some sources may make it sound as though it is about 6.5 miles from the Scenic Drive to Red Rock summit, but I clocked it at 5.1. Also, although directions often mention a sign, there was no longer one as of July 14, 2012.
I mention this because there is a possibility that not knowing it could get someone unfamiliar with the location in trouble. Because the sign is not there, one might pass Red Rock Summit at 5.1 miles and think it is the beginning of yet another dip before climbing to the true summit 1.4 miles later. The danger is that less than a mile from RR Summit, the road becomes so nasty that it would be difficult for someone in a stock SUV or truck to get through, and a stubborn or foolish person might get stuck or incur serious vehicular damage in trying to continue to where he thinks the trailhead will be.
The road is really rough. I definitely needed 4wd to get through in a few spots, and the old 4Runner I was in had higher clearance than just any stock SUV you see these days. Some people in Explorers were turning around. I'd recommend skid plates in addition to high clearance and 4wd, though I only scraped bottom once.