Dear Mr. Summers,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located a series titled Interstate System Planning and Designation Files, 1939 - 1989 in the Records of the Federal Highway Administration (Record Group 406) that may include the information you seek. For access to these records, please contact the National Archives at College Park - Textual Reference (RDT2) via email at firstname.lastname@example.org.
We hope this is helpful. Best of luck on your research!