[ubernostrum]

Being able to sense the design speed of a road is completely unrelated to someone's estimation of their driving skills.

The design speed of a road is, in fact, something you're probably only partly consciously aware of as you're driving on it; it's something you get not from a speed-limit sign but from cues like the way curves are built, the length of merging ramps and exit lanes, or how far ahead of something a warning sign is posted. Whether you think you're an above-average driver has nothing whatsoever to do with the fact that you will, consciously or not, learn to pick up on those cues and you will adjust your driving to it unless you're constantly watching your speedometer (and even if you do, if you drive significantly below the design speed, there will be times when you're uncomfortable doing so, though you might not realize why it's making you uncomfortable).