U.S. Route 51 in Illinois

This article is about the section of U.S. Route 51 in Illinois. For the entire route, see U.S. Route 51.

U.S. Route 51 (US 51) in the U.S. state of Illinois, is a main north–south artery that runs from the Ohio River north to the Wisconsin border, a distance of 415.95 miles (669.41 km).^[1]

U.S. Route 51
US 51 highlighted in red, former alignment in blue
Route information
Maintained by IDOT and ISTHA
Length	415.95 mi[1] (669.41 km) Length may include both directions of route where it is routed along one-way streets.
Existed	1926–present
Major junctions
South end	US 51 / US 60 / US 62 near Wickliffe, KY
	I-57 / IL 3 near Cairo I-64 near Richview I-70 at Vandalia I-72 at Decatur I-55 / I-74 at Bloomington I-39 / I-55 at Normal I-80 at LaSalle I-88 Toll / IL 110 (CKC) at Rochelle I-90 Toll at Cherry Valley I-39 / I-90 at South Beloit
North end	US 51 at the Wisconsin state line in South Beloit
Location
Country	United States
State	Illinois
Counties	Alexander, Pulaski, Union, Jackson, Perry, Washington, Jefferson, Marion, Fayette, Shelby, Christian, Macon, DeWitt, McLean, Woodford, Marshall, LaSalle, Lee, Ogle, Winnebago
Highway system
United States Numbered Highway System List Special Divided Illinois State Highway System Interstate US State Tollways Scenic
← IL 50 → US 52

US 51 enters Illinois from Kentucky at Cairo, Illinois. The highway heads northbound to Mounds, near Cairo, and begins to overlap Interstate 57 (I-57), following it for 24 miles (39 km) to Dongola, before splitting and heading north. The highway remains two lanes wide from Dongola to Assumption with the exception of a 10-mile (16 km) section between Centralia and I-64.

Past Assumption, US 51 becomes an expressway to Decatur. In Decatur, US 51 follows I-72 to bypass the city. US 51 leaves I-72 after eight miles (13 km), and it heads north to Bloomington–Normal as an expressway. At Bloomington–Normal, US 51 follows I-74 for a mile, then I-55 for seven miles (11 km), before following I-39 for 140 miles (230 km).

US 51 follows I-39, intersecting I-80 and I-88 along the way. The highway also follows US 20 south of Rockford (while still following I-39). I-39/US-51 join I-90, making US 51 one of the few toll highways in Illinois that is a U.S. Highway. US 51 exits I-39/I-90 just a mile south of the Wisconsin state line. Then US 51 follows Illinois Route 75 (IL 75) west to the intersection of IL 251, and then turns north through South Beloit, Illinois to enter Wisconsin.

US 51 was established in Illinois in the 1920s. Over the years, it became a heavily traveled highway, often experiencing many accidents, particularly in the northern half of the state. This highway gained the moniker of "Killer 51". As a result of these problems, Illinois pushed for a new four lane highway along the corridor. One was proposed in the 1960s and 1970s as part of a proposed supplemental freeway system, but only the area between Rockford and Decatur was given a high priority.^[2]

In the 1980s and early 1990s, US 51 was rebuilt to Interstate standards between Rockford and Bloomington–Normal on a new alignment. This new highway became Interstate 39, and US 51 was rerouted onto it. In addition, north of Rockford, US 51 was rerouted onto the existing I-90 segment between South Beloit and Cherry Valley. Most of the old highway became IL 251.

South of Bloomington–Normal to Decatur, US 51 remained largely on the original alignment, and it was expanded to an expressway. In the 2000s, the expressway was continued southward from Decatur to Assumption, Illinois.

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