Interstate 39 (I-39) is a highway in the midwestern United States. I-39 runs from Normal, Illinois at I-55 toWisconsin Highway 29 (WIS 29) in Rib Mountain, Wisconsin, approximately six miles (9.7 km) southwest ofWausau.^[2] 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.

In Illinois, the route has a total length of 140.82 miles (226.63 km).^[3] In Wisconsin, I-39 has a distance of 182 miles (293 km). With the exception of an eight-mile (13 km) segment around Portage, the Interstate shares a route with at least one other route number in the state. From Rockford to Portage, I-39 is concurrent with I-90.I-94 joins the pair in Madison until Portage. At 29 miles (47 km) in length, this concurrency of three Interstates is the longest in the country. From Portage northward, US 51 is co-signed with the Interstate and has exit numbers based on its mileage.

South end:	I-55 / I-74 / US 51 in Normal, IL
	I-80 in LaSalle, IL  I-88 / IL 110 (CKC) inRochelle, IL  I-90 in Rockford, IL  I-43 / WIS 81 in Beloit, WI  I-94 / WIS 30 in Madison, WI  I-90 / I-94 / WIS 78 inPortage, WI
North end:	US 51 / WIS 29 in Rothschild, WI

Source: http://en.wikipedia.org/wiki/Interstate_39