Illinois Route 73Illinois Route 73 is a north-south state highway in northwest Illinois. It runs from the U.S. [Route 52]/Illinois Route 64 concurrency south of Lanark north to the Wisconsin border north of Winslow. This is a distance of.Route descriptionIllinois 73 is a two-lane surface road for its entire length. At its northern terminus, Illinois 73 becomes County Highway M in Green [County, Wisconsin]. This is the only Illinois state highway that does not either continue as another state highway, or end at a ferry crossing.HistorySBI Route 73 existed from Illinois Route 72 north of Lanark to Winslow. Around 1982, it replaced Illinois 72 from north of Lanark south to U.S. Route 52.