When I said bypass, I meant not just for Chicago, but for all the north central IL cities mentioned.
Yet I-39's as-built route leads it right into Bloomington - Normal, instead of coming at a location that could be extended southward to I-74 and later to I-72.
I disagree about the Belvidere to Pontiac freeway, it would be pointless.
It would serve a greater population than as-built I-39 does, provide better access to Northern Illinois University, but still travel through an area with light enough development to function as a bypass.
Where would it come out on both ends?
I don't see anything making it that hard to tie into I-55 east/north of Bloomington-Normal. North end could have easily been at I-90 in the 17 mile gap in exits between the Belevidere and Hampshire interchanges.
Might as well shift it further east and make it the Praire Parkway.
Would be too close to Chicagoland to work well as a bypass; it would start getting traffic from the developing areas of Yorkville and Sugar Grove. Any growth around DeKalb-Sycamore, Ottawa, and/or Streator would be smaller and less likely to degrade the reliability of the route as a bypass.
I could see the Rockford to Lincoln freeway along U.S 51 (north of I-80), I-180/IL-29 and I-155, but if you are going to do that, you would need to build a leg to Decatur (which really suffered when I-39 was not extended to it). What really should have been done is I-57 should have been built straight north towards Decatur and ran along the I-72 alignment to Champaign-Urbana to its current alignment. Then build the Rockford-Peoria-Decatur variant of I-39 and you're set.
So what makes Decatur so worthy of having a north-south freeway? I'm sure Danville could do better had it gotten a north-south freeway. A Pontiac to Belvidere freeway would have served almost as many people as there are in Decatur.