I-39 from Rockford to Bloomington is by far the WORST maintained road I have ever traveled. The past 3 weeks I took a road trip through 9 states and never encountered anything like this anywhere else. There were potholes from the entrance ramp south of Rockford to the merge with I-55 by Bloomington. I just kept praying my tires would survive. It looked just as bad on the other side of the road going north.
The section from Rockford to I-88 at Rochelle is the oldest.(1984) Then the section from Rochelle to I-80 was built. (1986) After the Lincoln bridge opened over the Illinois River at LaSalle-Peru, it ended at IL-251 (former US-51) (1987)
When the bridge at the Illinois River was being done, the exit ramps at I-55 (Bloomington) were being done (1989) as well. But that section ended at Ziebarth Road (BUS US-51) and you used the existing 2 lane US-51.
Then construction went into this off again, on again phase where funding & planning ran into issues. It took several more years for IDOT to complete the route.
The northbound route got extended from Ziebarth Road to south of Kappa. It took an incredibly long, long time for the contractor to get the Mackinaw River bridge done. I remember driving by there how many times and saw very little progress. They would route you over to the 2 lane bridge (IL-251) so you could see up close what they were doing. It's an urban style design today with no median.
Just before the bridge over the Mackinaw was finished, they got the contracts issued for the rest from Kappa up to IL-251 at Oglesby.
So the last section completed was the one between Minonk and El Paso. Everyone all remembers the "Wenona Weave" where they would re-route US-51 over to the freshly poured ROW, then back over to the old ROW. It was the longest part built right on top of the old US-51.
It took 5 solid years to get the section between Oglesby and Normal finished.
Why?
The highway from Oglesby south to Decatur was recommended to be a four-lane divided expressway, utilizing the existing road where possible. After a decade of lobbying and back and forth debate, Governor James R. Thompson announced in 1986 that US 51 would be rebuilt to Interstate standards from Oglesby to Bloomington–Normal.[11]
However, due to funding concerns and intense local opposition, it was decided that US 51 would not built to Interstate standards between Bloomington and Decatur.[12] The Bloomington-Decatur segment was instead upgraded to a four-lane divided expressway, with mostly at-grade intersections, just as the transportation committee proposed. To this day, there remains ramp stubs northwest of the city along I-72, where the highways were to connect.[13]
So based on the years of construction, I can understand why any part of I-39 between Rockford and I-88 would be in the worse shape. It is the oldest built in 1984. (34 years) But bad road from Oglesby to Kappa is a surprise as it is 8 years younger (1992)
If IDOT followed its usual pattern, they paved asphalt over the original concrete to extend its life. But the base underneath is breaking up and causing the asphalt to fracture and breakup, hence the potholes.
If memory serves IDOT was only specifying an 8 inch concrete layer back then. For reference the Tollway specified a then record 12 inch pour depth on I-355 at the same time. (I think they are up to 16 now).
I remember bringing up the 12 inch pour the tollway used with an engineer from Kansas DOT. The first thing he said was "what are you doing, landing planes?....that is an airport depth!"
So all things considered the route is past or about to reach the end of its design life.