It carries over 10000 a day what can I say?
I don't doubt you, I just can't find a reason.
Part of it goes to Beardstown and the other part goes to Havana.
Both declining river cities.
Beardstown has JBS Meat Processing. But already has rail access, so its not like they are trucking down to transload.
The only other type of traffic I can come up with is Macomb based. Its out of the way to go I-74 via Peoria and I-72 via Quincy.
Havana is about to lose its longtime coal fired power plant.
There isn't any significant industry heading in/out of Springfield to the NW other than the New Salem park.
The general population shift in Springfield has been south (Chatham-Glenwood) or west (Washington Street/Old US 36)
I will have to look at the studies and see where the traffic is coming and going.