Illinois Route 173 is an east–west state road that runs from Illinois Route 251 in the Rockford suburb of Machesney Park east to Illinois Route 137 (Sheridan Road/Buckley) in Zion near the Illinois-Wisconsin border; this is a distance of 66.20 miles (106.54 km). Illinois Route 173 is also one of the northernmost east-west state highways in Illinois. On Interstate 94 (Tri-State Tollway) traveling south from Wisconsin, it is the second road seen which has an interchange at that location. Similarly, when traveling south/east from Wisconsin on Interstate 39/90 (Jane Addams Memorial Tollway), Illinois Route 173 is only the third exit into Illinois. In 2008, the entirety of Illinois 173 has been designated the 173rd Airborne Brigade Highway

Illinois 173 travels across relatively hilly northern Illinois from the Rockford area to Zion. This area is more densely populated than other portions of rural Illinois.

In July 2007, a new interchange was constructed at Illinois 173 and Interstates 39/90 (Jane Addams Memorial Tollway).

From east to west, Illinois Route 173 is known by local road names through the towns it traverses.

SBI Route 173 originally ran from near downtown Rockford (along Forest Hills Road) to Zion. During the 1980s, when the village of Machesney Park was heavily developed and incorporated, Illinois Route 173 was scaled back and its western terminus moved to Illinois Route 251 (North 2nd Street). From 1972 to 1994 the highway also assumed a portion of what is now Illinois Route 137 north to the Wisconsin border. 3