The road Interstate 39 replaces has a junction at Cairo Junction. It is over 1200 miles long.
 Cairo Junction is located at the junction of U.S. Route 51, Illinois Route 3, and Illinois Route 37 north of Cairo. 
U.S. Route 51 is a south-north United States highway that runs for 1286 mi from the western suburbs of New Orleans, Louisiana to within 100 ft of the WisconsinMichigan border. 
 I-39 was designed to replace U.S. Route 51 (US51), which in the early 1980s was one of the busiest two-lane highways in the United States. 
