porsche is already working on this. i posted an article last year on it. they're calling it "acc innodrive" (adaptive cruise control, innovative drive) GPS maps out the route, it takes things like corner radii, traffic, elevation changes, posted speed limits, etc. and the computer manages throttle, gear changes, etc. (though currently you get to steer)...
http://986forum.com/forums/general-discussions/29496-porsche-acc-innodrive.html