and youre right in many respects. posted speeds can change based on input. also a straight roadway technically could be "designed" for a million miles per hour. whereas curves are designed based on a agreed upon "design speed". New roadways do have to set a design speed and the road is designed for that speed. a certain radius curve is set for a certain speed based on the friction forces the avg car/tire can reasonably handle.

Yes we do design roads for a specific design speed and set the limit accordingly but there is wiggle room there.