High Quality Content by WIKIPEDIA articles! Interstate 39 (I-39) is a highway in the midwestern United States. I-39 runs from Normal, Illinois at Interstate 55 to Highway 29 in Rothschild, Wisconsin, approximately six miles south of Wausau. I-39 was designed to replace US Highway 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.