>> Most highways are underposted and should be 70-90.
Maybe in the middle of Montana or Texas where it's 50 miles between towns, but not anywhere with off/onramps. People are very bad at merging and it'd be very difficult for many vehicles, especially trucks to get up to speed if 90 was the limit.
That's what on-ramps are for. If traffic is congested enough to demand merging (rather than just making sure the road is clear), you shouldn't be driving at 90mph. On the other hand, you probably shouldn't be driving at 60 either. Speed limits should be limits, not averages, or worst-case.
You make a good point when it comes to merging. Speeds of 90-120 mph are reasonably safe on German autobahns, because people have a regimented driving style (absolutely no passing on right, but the left lane really is the fast lane) and follow it to a T. Most Americans aren't trained to drive those speeds, though, nor do they follow lane discipline.
Maybe in the middle of Montana or Texas where it's 50 miles between towns, but not anywhere with off/onramps. People are very bad at merging and it'd be very difficult for many vehicles, especially trucks to get up to speed if 90 was the limit.