Interstate 172 (I-172) is a spur route from I-72 entirely within the US state of Illinois. The highway runs north from its start outside of Hannibal, Missouri, to about 2.5 miles (4.0 km) west of Fowler. At U.S. Route 24 (US 24), I-172 becomes Illinois Route 336 (IL 336), which runs north and east to Macomb via Carthage. The entire portion of I-172 and I-72 from I-172 east to Springfield is also known by its former name, the Central Illinois Expressway. I-172 is 19.69 miles (31.69 km) long.