Illinois Route 21 is an arterial north–south state highway in northeastern Illinois. It runs from Illinois Route 43 in Niles to U.S. Route 41 north of Gurnee. Illinois 21 is 28.13 miles (45.27 km) long.

Illinois Route 43 is a 60.30-mile-long (97.04 km) major north–south state highway in the U.S. state of Illinois. It runs from U.S. Route 30 (US 30) in Frankfort north to the large intersection of IL 120 and US 41 in Waukegan.