When I took Drivers Ed we were informed of the “Basic Speed Limit”: The basic speed law requires drivers to operate vehicles at a safe speed for current conditions (weather, traffic, road quality), regardless of the posted limit. It dictates that you must not drive faster than what is safe, effectively making safety conditions take precedence over posted maximum speeds.
I don’t know if all states have that or not. Of course the safe limit is highly subjective. With my vehicles and their conditions and tires I can safely drive at higher speeds than some other vehicles. Just for example, I might drop to 5 MPH under the limit when other are at 15 under.