From 1918 through 1935, SBI Route 18 ran from Chicago to Princeton using what used to be part of U.S. Route 32. Since then, IL 18 as well as part of US 32 were superseded by US 34. In 1939, it was applied to what was then SBI Routes 89B and 89C as well as part of SBI Route 17. The next year, the route was completed and Illinois 17 moved to a new highway south of Streator.