Found this site that has links to lots of older documents and older Environmental Impact Statements:
http://onlinebooks.library.upenn.edu/webbin/book/lookupname?key=United%20States%2E%20Federal%20Highway%20Administration (http://onlinebooks.library.upenn.edu/webbin/book/lookupname?key=United%20States%2E%20Federal%20Highway%20Administration)
Worth noting that those links all go to HathiTrust pages which have been digitized by Google. Which means folded maps were not unfolded before they were digitized.
Still a lot of good stuff, including an EIS which indicates how much difficult it was to get the Beltline for Madision, WI finished (see page 14) (https://hdl.handle.net/2027/ien.35556030790943), plus an earlier EIS for the Beltline with a 3/4 cloverleaf and one semi-direct ramp for US 12 at US 51 (https://hdl.handle.net/2027/ien.35556030816680)