Illinois Route 172 is a short route shaped like an upside-down "L" on maps (so like the capital letter gamma). Its purpose is to provide access to Tampico, and the historic site home to Ronald Reagan's birthplace in that town. Almost all of it is in southern Whiteside County, south of Sterling-Rock Falls (the southern tip forays into Bureau County).
The northeastern end of Illinois 172 is at Illinois 40 about five miles south of Rock Falls.
From the terminus at Illinois 40, Illinois 172 heads due west, then turns due south toward Tampico:
Illinois 172 continues due south through Tampico until it ends at Illinois 92 in rural Bureau County.