Yeah. Other folks have already mentioned that the degree of enforcement leeway in the U.S. increased when the federal government made artifically-lower speed limits a requirement of federal highway funding in the 1970s. Which I can’t confirm or refute, but does make sense: I imagine that some states who disagreed with the change might have grudgingly set the formal limits in line with the federal policy, and then simply used lax enforcement to allow the speeds that they preferred all along. I have noticed that it’s often seemed politically unpalatable for officials to stick to a program of stricter enforcement to rein in a particular area’s entrenched driving culture after speed limits were increased in the 1990s, though.
In any case, if folks think that part of the reason for lax enforcement is measurement error then that could be used as an input toward designing a separate maximum speed designation. One could keep the “speed limit” enforceably defined in terms of the actual vehicle speed, while defining a new parallel “maximum speed” constraint strictly in terms of a measurement taken by law enforcement equipment that passes a particular calibration standard within a particular window of time before and after issuing the citation. Then you’d end up with one standard that gives the benefit of doubt on measurement error to the driver and another that gives the benefit of doubt to the enforcement record, and thus there’s a logical reason for (at least some of) the spread between those two thresholds. (This legal system might also make it easier to move toward maximum-speed enforcement that works more like existing license-plate-based tolling systems, allowing for a much more pervasive enforcement regime to push the culture toward compliance without the downsides of setting up lots of direct conflicts between irate drivers and law enforcement officers.)
Speeding is one of the most common ways for Americans to break the law. Drive the speed limit on the highway around here and you'll typically be the slowest car on the road. How much over the speed limit is customary varies regionally, but drivers often expect cops to ignore them at 5-15 mph over.
Overall, I think this is a pretty bad situation. It gets people used to ignoring laws, people who scrupulously follow the law are often at higher risk (and cause higher risk to those around them) than if they went along with traffic, driverless cars go awkwardly slow, some risk of selective enforcement, confusing for travelers, etc. How can we get out of this?
If we just started strictly enforcing the current limits we'd have a mess: it's too big a behavior change to push all at once so you'd see even more dangerous variance in speeds than today, and it's unclear we actually want people driving the posted speeds. It also wouldn't work well to raise the limit to the speed people are mostly going, since many people would assume they can then go an extra 5-15mph on top of that.
Instead we could take inspiration from Brazil and introduce a parallel system of maximum speeds:
Initially this has no legal effect, and just makes the existing amount of leeway more legible. On a 55mph road where people normally drive 60-65 and the police don't start ticketing until you're more than 10mph over, the signs would say both "speed limit 55" and "max 65". These would be rolled out gradually, in consultation with traffic engineers and the people responsible for enforcement.
As they roll out, you adjust enforcement to match. Put up speed cameras set to the maximum in many places, and in other places have police enforce the max strictly after each sign is put up. Traveling above the limit but below the maximum becomes effectively allowed, since there's no enforcement.
Once the rollout is complete you overhaul the laws around speeding to make the maximum the legal limit, and adjust rules that are set relative to the old limit to still make sense. For example, if you previously gave only low fines for going 58 in a 55 zone, and in practice never issued them, while you gave high fines for going 68, you would still want the higher fine for going 68 in a "max 65" zone. The goal is to bring the law in line with behavior, but otherwise keep the status quo.
At this point you could consider removing the older lower "speed limit" signs, but I think it's probably worth keeping them as advice about what speed to travel. In some cases you might raise them a bit, knowing that with the maximum in place as a firm limit you'll get slightly faster speeds but lower variance.
I think there's a path here that brings the law back in line with driver and enforcement behavior, while otherwise essentially maintaining the status quo. It does require new signs and some policy tweaks, but seems on balance pretty positive to me.