^Looking north from Exit 104, the interchange with Illinois 64, which is between Oregon and Sycamore, February 2024.
Interstate 39 runs through the backbone of north-central Illinois, starting at Bloomington-Normal and heading north to Rockford and beyond. It spends about 140 miles within the state of Illinois before crossing into south-central Wisconsin. It is concurrent with US Route 51 for its entire length in Illinois (save for about 1000 feet right before the Wisconsin border).
Exit 27 accesses Minonk, as well as Woodford County Highway 2...
Exit 35 is for Illinois 17, which connects with Wenona and Lacon:
Exit 41 is for Illinois 18, which can be used to reach Henry and Streator:
Exit 48 provides access to Tonica, and LaSalle County Highway 54, which extends east, becoming LaSalle County 14 and taking travelers to the southern end of Illinois 178:
Exit 51 reaches Illinois 71 with access to Oglesby and Hennepin:
Exit 52 is for Illinois 251 which crosses from southeast to northwest, and "LaSalle-Peru" is labeled as the pair of control cities:
Exit 54 is a diamond interchange with access to Oglesby via LaSalle County Highway 23:
Exit 57 provides access to US Route 6, LaSalle-Peru, and Ottawa:
Exit 97B is the latter ramp accessing Interstate 88 west, toward Moline-Rock Island:
Exit 99 reaches out to Illinois 38, with access to Rochelle and DeKalb:
Exit 104 is for Illinois 64, with access to Oregon and Sycamore:
Exit 111 is for Illinois 72, and though Monroe Center is right next to the interchange, Byron and Genoa are the only control cities listed on signs:
Exit 115 is for Baxter Road, on the southern outskirts of the Rockford area:
On the southern flank of Rockford, Interstate 39 and US Route 51 both join the US Route 20 bypassfreeway eastward; though no exit number is shown, the exit number is probably "119":
Exit 122A is the first half of a cloverleaf interchange with Harrison Avenue, and at this first exit, US Route 20 splits away to continue east toward Belvidere:
Exit 122B, at the second ramp of the cloverleaf, puts drivers on Harrison Avenue westward into Rockford:
At another apparently unnumbered exit (we'll call it "123"), Interstate 39 can access Interstate 90 east, but Interstate 90 joins Interstate 39 north here: