It’s obvious that many drivers routinely exceed the speed limit in many areas, not just on freeways. So the next logical question is: How are speed limits determined? One would hope that there is some science, not just politics or customary historical norms, that inform the speed limit decision for a stretch of road.
Your hope is not unfounded. The law does set default speed limits for various kinds of roads, which I guess you could say is setting limits based on historical norms, and they’re set in state statute by elected officials, so you could call that politics, but there’s more to it than that.
We don’t build roads however we want and then hope the default speed limit works for them. There are design standards (lots of them) that apply depending on the size and speed of a road. Let’s say that a city road engineer is designing a new road. State law sets the maximum limit for a city road at 25 mph. If that’s what the city needs, the engineer would design a road that meets the standards for that speed limit. But maybe the city has in mind an arterial with higher speeds. The law allows increases and decreases in speed limits “on the basis of an engineering and traffic investigation.” If the road is designed for it, the city could have a higher speed road. Apologies to the road engineers for oversimplifying here, but that’s my layman’s summary.
A speed limit could also change after a road is built. As homes, businesses, and driveways increase along a stretch of road, a city might do a study and decide to reduce the speed limit based on factors that weren’t present when the road was built. The right speed limit is based on more than the design of the pavement; it also includes the surrounding conditions. For example, a road in proximity with lots of pedestrians should have a lower speed limit than a similar road where there’s rarely a walker or bike rider.