A recent post in the aaroads.com forum shared information that AASHTO (Association of American State Highway and Transportation Officials) have begun digitizing all of their records from 1920s to modern day. This includes not only applications but correspondences and other materials. There's a lot of good information - and I am sure over the upcoming months - there will be updates to websites like Mike Roberson's NC,VA,WV, and SC pages or in posts in groups like Freeway Jim on some of the new information. There's a lot and appears more to be added.
One of the more interesting finds so far is this map by the American Automobile Association (AAA) that shows the planned lane configurations of the original Interstate system. I was amazed by the amount of two lane interstates planned for 1971.
Interstate 95 in Maine, I-70 in Utah are well known. And over the years I have learned about I-25 in New Mexico and I-91 in North East Vermont but I-81 in South Central PA or I-26 in South Carolina is surprising. Granted, I am looking at it in a modern era, but it is pretty interesting to see.
The AASHTO Route Numbering Archive can be found here.