Interstate 39 is a freeway serving the US Route 51 corridor in the southern 60 percent of Wisconsin and the northern 40 percent of Illinois.  It hugs a vertical centerline through each state, for as long as it runs.  The northern end is in Wausau, Wisconsin, where the freeway carrying Wisconsin Highway 29 continues west away from the city.  (Highway 29 joins along to the south for a bit before continuing eastward as a freeway.)  Interstate 39 and US Route 51 continue south to Portage, Madison, Janesville, and Beloit in Wisconsin.  From Portage to Madison, Interstate 39 is joined by Interstate 94; from Portage into Illinois, Interstate 39 is concurrent with Interstate 90, which departs from Interstate 39 at Rockford, Illinois.  (Note that the section of Interstate 39 concurrent with Interstate 90, from South Beloit to Rockford, is tolled.) US Route 51 is concurrent with Interstate 39 for every mile the Interstate traverses in Illinois.  The last of these miles meets Interstate 55 on the northside of the Bloomington-Normal area (so in Normal, since it's the northerly of the two cities).  Interstate 74's interaction with Interstate 55 is only a mile to the west from this southern terminus.

My photo of an Interstate 39 marker comes from the northernmost interchange Interstate 39 reaches in Illinois.  The interchange is with US Route 51 and Illinois Route 75. At this point, Interstate 39 is duplexed with Interstate 90.  Traffic exiting the truck stop to the north, onto Illinois Route 75, is greeted by this sign showing them how to get back to the Interstate. The aperture faces south. This photo was taken along the earliest part of my drive to the Pacific Ocean by way of Glacier National Park, in August 2018.
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