I-39 highlighted in red | |||||||
Route information | |||||||
Maintained by IDOT, ISTHA and WisDOT | |||||||
Length | 306.14 mi[1] (492.68 km) | ||||||
Existed | 1986–present | ||||||
NHS | Entire route | ||||||
Major junctions | |||||||
South end | ![]() ![]() | ||||||
| |||||||
North end | ![]() ![]() | ||||||
Location | |||||||
Country | United States | ||||||
States | Illinois, Wisconsin | ||||||
Counties | IL: McLean, Woodford, Marshall, LaSalle, Lee, Ogle, Winnebago WI: Rock, Dane, Columbia, Marquette, Waushara, Portage, Marathon | ||||||
Highway system | |||||||
|
Interstate 39 (I-39) is a highway in the Midwestern United States. I-39 runs from Normal, Illinois, at I-55 to State Trunk Highway 29 (WIS 29) in the town of Rib Mountain, Wisconsin, which is approximately six miles (9.7 km) south of Wausau.[2] I-39 was designed to replace U.S. Route 51 (US 51), which, in the early 1980s, was one of the busiest two-lane highways in the United States.[citation needed] 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). From Rockford, Illinois, to Portage, Wisconsin, I-39 runs concurrently with I-90. In Wisconsin, 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 cosigned with the Interstate and has exit numbers based on its mileage.