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.
In Illinois, I-39 begins at I-55, north of Bloomington–Normal, Illinois. I-39's southern terminus is less than one mile (1.6 km) from I-74 as I-74 runs around the city of Normal. From the city, I-39 runs north largely through rural areas, intersecting Illinois Route 251 (IL 251) at exit 8. About 55 miles (89 km) north of the city, I-39 crosses the Illinois River over the Abraham Lincoln Memorial Bridge, which is 1.3 miles (2.1 km) long.[4] Just north of the Illinois River, I-39 runs east of the cities of LaSalle and Peru before intersecting I-80 at exit 59. North of I-80, the wind turbines of the Mendota Hills Wind Farm can be seen from milepost 72 at Mendota north to near Paw Paw. I-39 intersects I-88 at exit 97 in Rochelle. Further north, I-39 crosses the Kishwaukee River before meeting US 20 on the southside of Rockford. I-39 then runs east concurrently with US 20 to where the Interstate joins the Jane Addams Memorial Tollway and I-90 near Cherry Valley. While concurrent, I-39/I-90 follows I-90's exit numbering. I-39/I-90 heads north to South Beloit. There is a toll plaza just south of Rockton Road. US 51 leaves I-39/I-90 at IL 75 at exit 1 in South Beloit, while I-39/I-90 continues north into Wisconsin.
For all but one mile (1.6 km) that I-39 is in Illinois, it is designated concurrently with US 51.
Wisconsin
I-39 enters from Illinois along with I-90, passing under County Trunk Highway P (CTH-P, Stateline Road), and bypasses Beloit to the east. East of the town, the route has an interchange at exit 185 that serves as the terminus for both WIS 81, which heads westward into Beloit, and I-43, which provides access to Milwaukee.[5] The last interchange serving Beloit is CTH-S (Shopiere Road) at exit 183. The I-39/I-90 concurrency continues to the north and is joined by WIS 11 about seven miles (11 km) north of the I-43 interchange. WIS 11 leaves I-39/I-90 at exit 175, heading east. The highway bypasses Janesville to the east, although interchanges with US 14 and WIS 26 (Milton Avenue) provide access to the town. The highway continues generally to the north, crossing the Rock River before an interchange with WIS 59 that provides access to Edgerton to the west and Milton to the southeast.[5] Subsequently, the highway enters Dane County as it passes west of Lake Koshkonong. It is joined at exit 160 by US 51 from Edgerton and serves as the southern terminus of WIS 73. US 51 leaves the route four miles (6.4 km) to the north and heads west through Stoughton. The Interstate gradually turns westward around Utica to an interchange with CTH-N, providing access to Stoughton and Cottage Grove. It then turns gradually back to the north and meets US 12 and US 18 in Madison. I-39/I-90 bypasses Madison to the east, and I-94 joins the concurrency at the eastern terminus of WIS 30, an interchange known as the Badger Interchange.[6] About two miles (3.2 km) to the north, the highway crosses US 151, which includes a south side access to High Crossing Boulevard. The last two Madison area interchanges are US 51 three miles (4.8 km) northwest of the US 151 interchange and WIS 19 another one mile (1.6 km) northwest of the US 51 interchange. Access is provided to CTH-V (West North Street) via exit 126 just west of DeForest four miles (6.4 km) further north. I-39/I-90/I-94 enter Columbia County four miles (6.4 km) north-northwest of CTH-V.[5]
I-39/US 51 in northern Wisconsin
The Interstate crosses WIS 60 at an interchange three miles (4.8 km) north of the county line east of Lodi and CTH-CS at another interchange four miles (6.4 km) further north near Poynette. The highway crosses the Wisconsin River four miles (6.4 km) north of CTH-CS. At three miles (4.8 km) further along the route from the river, I-39 leaves the concurrency with I-90/I-94 at exit 108 and turns northward while the other two Interstates turn northwesterly. WIS 78 terminates at this interchange and heads southwest. This is the starting point of the segment of freeway that carries the I-39 route alone. The Interstate crosses WIS 33, the first of three interchanges accessing Portage, two miles (3.2 km) north of I-90/I-94. After crossing the Wisconsin River again, I-39 crosses the second interchange—this one with WIS 16—and turns northeasterly to an interchange with US 51. The U.S. Highway joins the Interstate, and both turn north once again and leave the Portage area and, after four miles (6.4 km), enter Marquette County.[5]
WIS 23 joins I-39/US 51 northbound, four miles (6.4 km) from the county line. The highway passes along Buffalo Lake and encounter a southside half interchange with CTH-D in the town of Packwaukee. WIS 23 leaves the concurrency to the east heading toward Montello at WIS 82 near Oxford, and the freeway takes a due north route to pass Westfield. I-39/US 51 enters Waushara County six miles (9.7 km) north of Westfield.[5] Four miles (6.4 km) north of the county line, I-39/US 51 junctions with WIS 21 in Coloma. I-39/US 51 meets an interchange in Hancock with CTH-V five miles (8.0 km) further north, and WIS 73 crosses in Plainfield after another five miles (8.0 km). This is two miles (3.2 km) south of the Portage County line.[5] In Portage County, I-39/US 51 take a due north trajectory, which provides access to CTH-D, CTH-W, and WIS 54 (also Business U.S. Highway 51 (Bus. US 51)) over 12 miles (19 km). The WIS 54 interchange and the CTH-B interchange 1.5 miles (2.4 km) north of it provide access for Plover. The next four interchanges—CTH-HH, US 10 eastbound/WIS 66 westbound, Stanley Street, and Bus. US 51—provide access to Stevens Point. Among these interchanges, the freeway turns northwestward, bypassing the city to the east and north. I-39/US 51 crosses two more interchanges while in Portage County: Casimir Road four miles (6.4 km) northwest of Stevens Point,[7] and westbound US 10 two miles (3.2 km) north of Casimir Road. The freeway then parallels the Wisconsin River for six miles (9.7 km) to an interchange with CTH-DB east of Lake DuBay and one mile (1.6 km) south of the Marathon County line.[5]
Northern terminus of I-39 at the interchange with WIS 29 near Wausau
WIS 34 terminates at an interchange with I-39/US 51 in Knowlton three miles (4.8 km) northwest of the freeway's entry into Marathon County; the freeway turns due north from this interchange. WIS 153 crosses the freeway four miles (6.4 km) further north in Mosinee. Maple Ridge Road crosses after another two miles (3.2 km) as the freeway turns northeastward into Kronenwetter.[8] An interchange with Bus. US 51 is just south of the Wisconsin River crossing after four miles (6.4 km) from Maple Ridge Road. I-39 ends two miles (3.2 km) further north at the interchange with WIS 29 east just southwest of Wausau. US 51 continues north on the freeway after WIS 29 toward Merrill.[5]
History
Illinois
When the Interstate Highway System was first being planned, Illinois made a request for a north–south highway from South Beloit to Salem. The project was deemed a low priority and was shelved. US 51, which ran mostly down the middle of the state, became a heavily traveled two-lane arterial road, experiencing many crashes and earning the nickname "Killer 51".[9]
In the late 1960s and early 1970s, a major supplemental freeway system plan was proposed, with the goal of providing Illinois residents access to freeways within 30 minutes or less. One of the proposed routes, FAP 412, was a route that would extend from US 20 in Rockford to I-57 just north of Salem, similar to the earlier requested route. Due to traffic counts, only the portion between Rockford and Decatur was prioritized.[10]
Over the course of the 1970s, planning for the US 51 supplemental freeway took place in earnest. However, debate ensued over what type of highway should be built. The Illinois Department of Transportation (IDOT) wanted the entire highway built to Interstate Highway standards, but a transportation committee established to review the proposed supplemental freeway system recommended only Interstate construction between Rockford and I-80.[11] The highway from Oglesby south to Decatur was recommended to be an at-grade expressway, utilizing the existing road where possible. After a decade of lobbying by interest groups, it was announced in 1986 that US 51 would be rebuilt to Interstate standards from Oglesby to Normal.[12] However, due to funding concerns and local opposition, it was decided that the Bloomington to Decatur segment would not be built to Interstate standards; this segment was made a four-lane expressway.[13]
The first segment of the freeway opened 1984 from IL 5 (now I-88) in Rochelle, to US 20 in Rockford.[14] When the freeway was completed south from IL 5 to I-80 in 1986, IDOT officially requested an Interstate designation for the new highway, and I-39 was officially designated.[15] By December 1987, construction on the section of I-39 between I-80 and IL 251 was finished. The next section, between IL 251 and I-55 in Bloomington–Normal, was completed by 1992, although this stretch of the highway was opened in several phases as completed.[16] In December 1989, the section from Bloomington–Normal to Hudson opened, a distance of about 4.4 miles (7.1 km).[17] In early September 1992, another segment opened from IL 116 north to IL 17.[18]
Wisconsin
In Wisconsin, the highway was officially designated in 1992.[19] In October 1993, AASHTO established part of I-39 in its northern section between Rockford and Rib Mountain, Wisconsin, then designated I-39 along existing portions of I-90, I-94, and US 51. However, this part of the highway was not marked as I-39 for another four years, primarily because the Wisconsin Department of Transportation (WisDOT) had to reconstruct the interchange connecting I-90 and I-94 with WIS 78 near Portage.[20] Signs denoting I-39 were placed along the highway in Wisconsin until 1996, when the section between Portage and Rib Mountain (near Wausau) received its signs.[19] This occurred after then–Governor Tommy Thompson designated the stretch between Portage and Wausau in 1996 after a five-year push to get the Interstate designation approved.[20] The remaining segment along I-90/I-94 was not signed for I-39 until late 1998.[19] The section between the I-90/I-94 interchange and US 51's interchange in Portage was previously a part of WIS 78. That route was truncated back to its current terminus when the Interstate's designation went into effect.[21] The designation of I-39 violated Wisconsin's rule of not having any state trunk highway number duplicated—Interstate, US, or state—as WIS 39 already existed.[19]
^ abcdefghWisconsin Department of Transportation (2013). Official State Highway Map(PDF) (Map) (2013–2014 ed.). 1:823,680. Madison: Wisconsin Department of Transportation. Retrieved February 5, 2023.
^Illinois Department of Transportation (April 12, 1978). Priority Primary Routes(PDF) (Map). Illinois Department of Transportation. Retrieved March 23, 2010.
^Wills, Bill (August 27, 1976). "Sides Square Off in Hearings on Freeway". The Pantagraph. Bloomington, Illinois. p. 3. Retrieved September 12, 2022 – via Newspapers.com.
^Featherly, Marc (March 11, 1986). "Close Encounters on US 51 Trip". The Pantagraph. Bloomington, Illinois. pp. A1, A5. Retrieved September 12, 2022 – via Newspapers.com.
^Weingroff, Richard (October 15, 2013). "Part I: History". The Dwight D. Eisenhower System of Interstate and Defense Highways. Federal Highway Administration. "Interstate Route Additions Under 23 U.S.C. 139(a) As of 12/31/97" (table). Retrieved March 7, 2015.
Southwest Region Staff (December 31, 2008). State Trunk Highway Log for Southwest Region (XLS). Madison, WI: Wisconsin Department of Transportation. 039N.
North Central Region Staff (December 31, 2008). State Trunk Highway Log for North Central Region (XLS). Rhinelander, WI: Wisconsin Department of Transportation. 039N.
^Wisconsin Department of Transportation. "Exit numbers on US 51 and I-39". Exit numbers on Wisconsin's freeways. Wisconsin Department of Transportation. Archived from the original on June 7, 2007. Retrieved June 30, 2007.
^Wisconsin Department of Transportation (December 31, 2017). Designated Freeways & Expressways(PDF) (Map). Scale not given. Madison: Wisconsin Department of Transportation. Retrieved April 4, 2020.
