Illinois Route 18 runs east-west where central Illinois meets north-central Illinois. It connects Henry, Streator, and really no other towns. It appears as a northern alternate to Illinois 17; I'm not sure if that has anything to do with its numbering as 17 plus 1. I drove this road from east to west in its entirety, so I am listing the junctions here starting in the east and going west.
Illinois 18's eastern end is a split with Illinois 17 west of Dwight:
Illinois 18 and Illinois 23 cross paths in Streator, with each route taking the form of a pair of one-way streets: