Check with the transportation institute at the University of Florida. Used to be called MacTrans. They actually sold the source code in the 80's on 5-1/4" floppy discs. I had it at one time but not sure I could find it now. Will try.
Also check with Shell in Europe and Chevron...both used Elsym5 as the basis for their pavement analysis programs....example...ChevPC is based on Elsym5 source code.