Interstate 172 connects I-72 & U.S. 36 with the Adams County seat of Quincy and the Illinois 336 expressway north to Macomb. The rural freeway passes east of Quincy along bluffs rising above the Mississippi River while the southern extent traverses the river flood plain in Pike County.
I-172 is also part of Illinois 110, the Chicago-Kansas City Expressway. Referred to as the CKC for short, the multi state route overlaps with other Interstates and U.S. highways as both an alternate route to Interstates 55 and 70 and an economic generator for cities located on the route.
IL 336 takes over from the ending Interstate 172 as an at-grade expressway north toward Carthage and east to Macomb. The expressway was cosigned as part of the CKC (IL 110) in 2010. Long range plans call for the IL 336 alignment to extend east beyond Macomb to Bardolph and Cuba; northeast around Canton; and east parallel to IL 116 to Interstate 474 at Bellevue. While incorporating some grade separations and interchanges, the overall route is not planned as an Interstate standard facility. More information is available at the Illinois 336 Peoria to Macomb Study web site.
Illinois originally submitted Interstate 172 as the new designation for the Central Illinois Expressway to AASHTO. The limited access highway was signed as U.S. 36 and completed in 1991 from I-55 at Springfield to Illinois 336 near Hull. AASHTO approved the numbering on June 9, 1991, with it contingent upon Federal Highway Administration (FHWA) approval. No signs were erected however, and by 1995 a new numbering plan emerged.
As approved by AASHTO on April 22, 1995, the Central Illinois Expressway was renumbered instead as a westward extension of Interstate 72. The I-172 designation was then applied to 17-mile long freeway section of Illinois 336 as it spurred north from the proposed I-72 to Quincy and U.S. 24.