The Illinois State Library has created a digital collection of 64 state highway maps dating from 1917 through 2008. You can access the collection at the Illinois Digital Archives site at <http://www.idaillinois.org/cdm4/browse.php?CISOROOT=%2Fisl9 [2]>. An article describing how the project was done can be found in the spring 2011 issue of the Bulletin of the Illinois Geographical Society (V. 52, No. 1). For additional information you may contact the ISL Map Librarians, Tom Huber, at thuber@ilsos.net [3].
Links:
[1] http://www.library.northwestern.edu/libraries-collections/evanston-campus/transportation-library
[2] http://click.icptrack.com/icp/relay.php?r=26222037&msgid=249136&act=EW4S&c=724286&destination=http%3A%2F%2Fwww.idaillinois.org%2Fcdm4%2Fbrowse.php%3FCISOROOT%3D%252Fisl9
[3] mailto:thuber@ilsos.net
[4] http://www.brookings.edu/events/2011/0518_energy_future.aspx
[5] http://www.library.northwestern.edu/related-pages/transportation