Interstate 39 (I-39) is a highway in the midwestern United States. I-39 runs from Normal, Illinois at I-55 to Wisconsin Highway 29 (WIS 29) in Rib Mountain, Wisconsin, approximately southwest of Wausau. I-39 was designed to replace US Highway 51 (US 51), which in the early 1980s was one of the busiest two-lane highways in the United States. I-39 was built in the 1980s and 1990s.