Many states in the US have the Basic Speed Law, e.g. California:

> No person shall drive a vehicle upon a highway at a speed greater than is reasonable or prudent having due regard for weather, visibility, the traffic on, and the surface and width of, the highway, and in no event at a speed which endangers the safety of persons or property.

The speed limit isn't supposed to be a carte blanche to drive at that speed no matter what; the basic speed law is supposed to "win." In practice, enforcement is a lot more clear cut at the posted speed limit and officers don't want to write tickets that are hard to argue in court.