IL 91 does not travel in a straight line between any two major towns. IL 91's endpoints are generally northwest-to-southeast, but the route consists entirely of north–south and east–west stretches.
SBI Route 91 was the current U.S. Route 150 from Knoxville to Peoria. Prior to 1936, Illinois Route 30 mostly followed present-day IL 91 and a part of IL 17 east of Galva.[4] Later, IL 30 was rerouted north of Toulon which entirely followed current IL 91.[5] In March 1941, the state announced that IL 91 was replacing the designation of IL 30 from Kewanee to US 150,[2] and has remained so to this day.[3][6]
^Illinois Secretary of State; Rand McNally (1936). Road Map Illinois (Map). [c. 1:950,000 and c. 1:1,110,000]. Springfield: Illinois Secretary of State – via Illinois Digital Archives.
^Illinois Secretary of State; Rand McNally (1939). Illinois Road Map (Map) (1939–1940 ed.). c. 1:918,720. Springfield: Illinois Secretary of State – via Illinois Digital Archives.
^Illinois Secretary of State; Rand McNally (1942). Illinois Road Map (Map). c. 1:918,720. Springfield: Illinois Secretary of State – via Illinois Digital Archives.