I've driven #18, the 550 in Colorado, several times. In a tractor/trailer. It's a white knuckler. There are many, many, MANY hair pin turns, and switchbacks. Steep climbs and drops. Quite a few that you simply CANNOT keep your trailer on your side of the road. One 90 degree turn, if missed, will send you over a cliff, and onto a town!!! In many places you are looking down at the tops of trees and telephone poles. It was as scary as it was beautiful.
That said, it would be a lot of fun on a motorcycle, or the X-Fire.