Speed limits on roads are almost always political decisions, not engineering ones. How else do you explain the straight as an arrow, flat as a pane of glass, brand-spanking-new, 60mph limit roads crisscrossing the american midwest?
Limiting speeds on roads that cut through farmland is probably a good idea. Imagine the damage that would be caused if a car hurtling down the rocketway at 90 intersected with a loose cow, tractor, or someone checking a mailbox that's perilously close to the road.
I'm talking interstates, not regular old roads. Last time I crossed the country by car (about a year ago) the average speed-limit was very low, absolutely not set at the safe driving speed. I forget what it was exactly, might have been more of an average at 65, but it was dramatically low.
Interestingly, iirc the exception to this was North Dakota. As I recall there were plenty of 70-80mph segments there (still low, but with the 5mph tolerance it was reasonable).
