It isn't always based on that. The CR nearby my house changes randomly and the cops hang at the change areas and catch people speeding.Speed limits are set based on terrain, visibility and traffic. I live on a County Rd, and the speed limit varies between 45 & 55 mph. There is a highway at one end that carries the same speed limits. There are plenty of places that a highway can drop the speed limit to 35. It depends on the locations.