here is an interesting pair of videos showing a 30s era limited access highway, compared to a modern highway, also regional difference.
The first is the Texas toll highway 130 featured on the above camaro video-
On this road, I think 85 definitely makes great sense as a limit, if not even faster-
Texas Toll Highway 130 North Between I-10 and Exit #458 Speed Limit 85 04/18/2013 - YouTube
However, heres a comparison road, this is highway 15 (merritt parkway) in CT, a late 30s design, despite constant straightenings and improvements, it remains a beautiful drive with no shoulders and sudden ramps. (video is sped up)
Driving east down the Merritt Parkway in Connecticut - YouTube
on this road the speed limit is 55. Now you can cruise at 70-75 all day, but if you look at 3.28 where a car gets on the highway, you can see why a 55 limit makes sense on such a road. There are a couple incidents in this video where someone getting on the highway causes some major braking