Interstate 39

Interstate 39
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	I-55 / US 51 in Normal, IL
Major intersections	I-80 in LaSalle, IL I-88 Toll / IL 110 (CKC) in Rochelle, IL I-90 Toll in Cherry Valley, IL I-43 / WIS 81 in Beloit, WI I-94 / WIS 30 in Madison, WI I-90 / I-94 / WIS 78 in Portage, WI
North end	US 51 / WIS 29 near Rothschild, WI
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 Highway System Main Auxiliary Suffixed Business Future
Illinois State Highway System Interstate US State Tollways Scenic
Wisconsin State Trunk Highway System Interstate US State Scenic Rustic
← IL 38 IL → US 40 ← WIS 38 WI → WIS 39

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.