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FreightWaves Classics/Infrastructure: I-39 serves rural Illinois and Wisconsin

This is the latest in a FreightWaves Classics series about the U.S. Interstate Highway System (IHS). The series began with a profile of Interstate 2 (I-2) and has covered each interstate in numerical order. Today’s article profiles I-39, which serves two states in the Midwest – Illinois and Wisconsin.

On February 17, a multi-vehicle “mass collision” occurred on I-39 near El Paso, Illinois. According to the Illinois State Police, the crash scene was several hundred yards long and involved about 100 vehicles. At the time of the series of wrecks, there was “wintry weather.” A 30-mile stretch of the interstate was closed because of the accidents. 


Primarily a route through rural areas, I-39 serves as a bypass around the Chicago and Milwaukee metropolitan areas. The interstate begins on the north side of Normal, Illinois, at I-55 and runs to State Trunk Highway 29 (WIS 29) in the town of Rib Mountain, Wisconsin (approximately 6 miles south of Wausau). 

I-39 runs for 306 miles. It is the 35th-longest interstate in the U.S. system and it is in the middle between the longest and shortest interstates. With the exception of an eight-mile segment around Portage, Wisconsin, I-39 shares its route with at least one other route number along its entirety. From Rockford, Illinois, to Portage, I-39 runs concurrently with I-90. In Wisconsin, I-94 joins I-39/I-90 in Madison and continues to Portage. This concurrency of three interstate highways is the longest in the nation (a total of 29 miles in length). From Portage northward, U.S. Route 51 (US 51) is cosigned with I-39. 

Construction on I-39 in Illinois began in the early 1980s.

History of I-39 Illinois

When the IHS was being planned in the early 1950s, Illinois made a request for a north-south highway from South Beloit to Salem. However, the project was considered a low priority and was not acted upon.  US 51 ran primarily north-south through the middle of the state. Over time, as traffic levels increased, it became a heavily traveled two-lane road. Ultimately there were so many crashes on the road that it earned the nickname “Killer 51.” 

A major supplemental highway system plan for Illinois was proposed in the late 1960s/early 1970s. Its goal was to provide Illinois residents access to a highway or highways within 30 minutes or less from their homes. One of the proposed routes would have extended from US 20 in Rockford to I-57 just north of Salem. This was similar to the interstate route that had been requested in the 1950s. Because of higher traffic counts, the section of proposed roadway between Rockford and Decatur was given priority.

During the 1970s, planning for the highway to supplement US 51 continued. However, there were discussions about what type of highway should be built. The Illinois Department of Transportation sought to have the entire highway built to interstate highway standards. However, a committee charged with reviewing the proposed supplemental highway system countered; it wanted interstate construction between Rockford and I-80 only. 

It was decided that the highway from Oglesby south to Decatur was to be an at-grade highway only, utilizing the existing roadway where possible. However, after a decade of lobbying by various groups, it was announced in 1986 that US 51 would be rebuilt to interstate standards from Oglesby to Normal. But due to funding concerns and local opposition, it was decided that the segment between Bloomington and Decatur would be built as a four-lane highway, but would not be built to interstate standards.

I-39 Overview Illinois

In Illinois, I-39 has a total length of 140.82 miles. With the exception of one mile of roadway in the state, I-39 is designated concurrently with US 51. The highway begins north of the Bloomington-Normal metro area at I-55. The interstate’s southern end is less than one mile from I-74 as I-74 travels around the city of Normal. From its intersection with I-39, Interstate 55 runs to either Chicago or St. Louis. I-74 South runs toward Indianapolis.

From Normal, I-39 runs northward, largely through rural areas. The highway crosses the Illinois River about 55 miles north of Normal via the Abraham Lincoln Memorial Bridge. The bridge is 1.3 miles long and has the distinction of being the state’s longest bridge. 

Just north of the Illinois River, I-39 runs to the east of LaSalle and Peru. It intersects I-80 at exit 59 and intersects I-88 at exit 97 in Rochelle. Continuing northward, I-39 crosses the Kishwaukee River and then meets US 20 on Rockford’s southside. I-39/US 20 travel east to the point where the interstate joins the Jane Addams Memorial Tollway and I-90 near Cherry Valley. While concurrent, I-39/I-90 run north to South Beloit. US 51 leaves I-39/I-90 in South Beloit. I-39/I-90 continue north into Wisconsin.

History of I-39 Wisconsin

Originally an intrastate interstate in Illinois, a proposal to extend I-39 into Wisconsin was made. The highway was officially designated in Wisconsin in 1992.That was followed by the American Association of State Highway and Transportation Officials (AASHTO) approving the extension of I-39 from Rockford, Illinois north to WIS 29 in Wausau, Wisconsin, on October 22, 1993. 

AASHTO then designated I-39 along existing portions of I-90, I-94 and US 51. But this part of the highway was not marked as I-39 for another four years, in large part because the Wisconsin Department of Transportation had to reconstruct the interchange connecting I-90 and I-94 with WIS 78 near Portage.

The designation of I-39 in Wisconsin violated the state’s rule of not having any state trunk highway number duplicated (whether by an interstate, US Route or state highway), because WIS 39 already existed.

I-39 Overview Wisconsin

In Wisconsin, I-39 is 182 miles long, which is longer than the highway’s length in Illinois. 

I-39 enters Wisconsin from Illinois concurrently with I-90, passing under County Trunk Highway P and bypasses Beloit to the east. East of Beloit, there is an I-39/I-90 interchange that is the end for both WIS 81 (which heads westward into Beloit), and I-43, which travels to Milwaukee. 

The combined I-39/I-90 bypasses Janesville to the east; however, there are two interchanges that access the town. The highway continues generally to the north, and crosses the Rock River. The combined highways enter Dane County, passing west of Lake Koshkonong. The highways gradually turn to the west around Utica to an interchange with CTH-N, which provides motorists access to Stoughton and Cottage Grove. The highways gradually turn northward again and meet US 12 and US 18 in Madison. I-39/I-90 bypass Madison to the east, and the highways are then joined by I-94 at the eastern terminus of WIS 30 (which is known as the Badger Interchange). 

The combined interstate highways cross the Wisconsin River. Three miles north of the river, I-39 leaves the concurrency with I-90/I-94 and turns northward; the other two interstates turn to the northwest. This is the start of the highway segment that carries I-39 alone. After I-39 crosses the Wisconsin River again, it turns northeasterly and an interchange with US 51. The highway joins the interstate, and both turn north. Four miles later, they enter Marquette County, pass through it and then run through Waushara County into Portage County, where I-39/US 51 take a due north route. The combined highway/interstate provide access to Stevens Point, but bypass the city to the east and north. 

I-39/US 51 parallel the Wisconsin River for six miles and then enter Marathon County. I-39 terminates at its interchange with WIS 29 East southwest of Wausau. US 51 continues north toward Merrill.