The National Maximum Speed Limit law was passed in 1974 and repealed in 1995. It's been repealed for longer than it was in effect. Virtually all of the lower 48 US states have an interstate speed limit on the order of 70-80 mph.
Yes, though a lot of them have 85th percentile speeds more than 5 mph above the speed limit. And they still have a tendency to randomly reduce the speed limit to 55 mph without any obvious difference in road design (Pennsylvania and Maryland for example). Also, a lot of those states only recently allowed speed limits above 65 mph (far more recently than 1995).
So you basically have a generation of motorists who grew up and spent a substantial portion of their driving years with highway speed limits that virtually no one complies with, and that it was okay to drive 5 to 10 mph above the speed limit. This attitude spread to other road types as well meaning that even appropriately posted speed limits (e.g., 25 mph residential streets) have a high degree of noncompliance.
It's going to take a while to undo the damage, but that's what happens when traffic control devices are effectively used to "cry wolf".
