It’s more complicated than that because several (most?) states have contradictory laws about impeding traffic. It can technically be illegal to drive at (or below) the speed limit because it creates an unsafe environment for all the other cars on the road that are driving faster, even if they’re all breaking the legal speed limit.
It’s not a viable defense if you get a ticket for speeding but in practice the speed limit is really the prevailing speed of traffic plus X mph, where X adjusted for the state. I.e. in my experience Texas is more strict about the speed limit even on their desolate highways, LA is about 10 mph faster than San Francisco, in Seattle it depends on the weather, you’ll never hit the speed limit in New York anyway, and in Florida you just say the gator ate the officer who pulled you over.