172 SHOULD kick in at 170 though, 150 isn't all that fast, and is perfectly common in states with higher speed limits like Texas (notice that 85mph speed limit section, I'll bet there's plenty of people going just over 90 there). Our roads are designed to handle these speeds. That said, the idea of speed limits is to keep it to a level everyone can be safe at, and in today's vehicles, that would be about 120km/h.
All that said, there are plenty of places where the limit is correct. A good number of places on the Gardiner I wouldn't want to drive or ride much above 90km/h anyway, and many residential areas are perfect; my street is a 40, and I'd be hard pressed to get there without slamming on the brakes before the next curve, it could seriously be a 30.