The Bloomington to Decatur segment was planned in the 1970s along with the Rockford to Normal segment. It was scaled back by 1978 or so to an expressway using the existing ROW due to concerns about farmland being taken out of production (Rick Powell, do you remember anything about this?). If they had waited a decade and used the design standards that were used between Oglesby and Normal, I wonder if they would’ve gone interstate on the Bloomington to Decatur segment?
I am not sure about farmland preservation being an issue on the Bloomington to Decatur segment (although I read USEPA's comments on the "bypass" alternatives vs. the "in town" alignments and they seemed to acknowledge the trade off in farm impacts vs. noise impacts depending on which alignment was chosen). But I do know that the Oglesby to Normal freeway section was designed with a 54 foot median and bridges with no center pier, in part to reduce the required farm acreage. The Bloomington - Decatur expressway was mostly a cost-saving move to get it done within the remaining statewide highway budget, from what I have gathered, although we did sneak in that US 136 interchange. A full access control probably wouldn't have taken that much more land, except for overhead structures every mile or two.
I worked on the in-house design plans for the first 4-lane segment going south of I-74; it must have been 1978 or 79; and was at the opening ceremony at Heyworth in the late 80s. I suspect that the segment I worked on pre-dated the EIS referenced above, since we were already doing final design well before 1981, and it was built well before the remaining sections to the south. Or maybe we were working on it so that it could be let shortly after the EIS was approved, since this segment had no alternatives.
The section through Clinton was considered for an in-town "one way couple" similar to Business 51 through Bloomington-Normal, but ultimately the west bypass was built with at-grade intersections.