Prompted by safety concerns at a major intersection near Morton, state transportation officials replaced the entire route with a limited-access freeway.

The highway passes to the west of Morton before intersecting I-74 at a trumpet interchange about eight miles (13 km) southeast of downtown Peoria.

[4] Surrounded mostly by prime land used for farming soybeans and corn, I-155 is a four-lane freeway through rural central Illinois.

The largest city directly served by I-155 is Tremont; however, the highway mainly carries traffic traveling to and from Peoria and Springfield.

In the early 1960s,[disputed – discuss] the Illinois Department of Transportation (IDOT) opened IL 98, an east–west, two-lane highway running west from downtown Morton to Pekin.

[3] Led by key supporters—US Representative Robert Michel, former IDOT transportation engineer Jack Harland, and pro-freeway organization "Route 121 by '91" chairman Jim Unland—IDOT initiated plans to upgrade IL 121 to a four-lane freeway.

In 1986, IDOT struck a deal with Swain, agreeing to reroute the road and take only 40 acres (16 ha) of land, ending the lawsuit and resuming construction activities.

[5] Illinois initially applied for the new freeway to be designated Interstate 37, but, on December 7, 1990, the request was deferred by the American Association of State Highway and Transportation Officials (AASHTO), pending approval by the Federal Highway Administration (FHWA) to add the freeway into the Interstate System.