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.

That law seems more likely to assign blame to drivers if they hit someone. So practically it's not enforced but in accidents it becomes a justification for assigning fault.