Interstate 155 provides a cut off between the I-74 corridor at Peoria and the I-55 corridor north from Springfield. The linear freeway travels a rural route north from the Lincoln area to Morton, replacing the old alignment of Illinois 121.
Illinois initially submitted the designation of Interstate 37 for the Illinois 121 corridor between Lincoln and Morton. AASHTO deferred on the designation at the Route Numbering Committee Meeting held on December 8, 1990, with the suggestion of a three-digit designation instead. The state resubmitted the route as Interstate 155 to AASHTO on June 9, 1991, and that designation was approved contingent upon the completion of the freeway route to Interstate standards.
Work on the $120-million project to upgrade Illinois 121 and construct Interstate 155 was finished on October 30, 1992. Initial discussion surrounding the upgrade of IL 121 to four-lane standards began in the early 1960s. The IL 121 intersection with newly built IL 98 gained the nickname “Killer Corner” when the two roads met south of Morton due 15 accident deaths that occurred at the intersection. That junction was replaced with an interchange in November 1989.1