I get the heebee-jeebies when I hit 30 or 35. But then the one time I did that, it was on a circular road around a park with a bridges with definite expansion bumps (danger of busting a tire) and, at the bottom of the hills, curves coming up. I'm not willing to risk wiping out at 40+ mph even if I am wearing a helmet and reasonably heavy (eg protective) clothing.