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).
Most people around here don't trust that the engineers who plan roads have some sense of what they're doing. :)