I-39 highlighted in red
|Maintained by IDOT, ISTHA and WisDOT|
|Length||306.14 mi  (492.68 km)|
|South end||I-55 / I-74 / US 51 in Normal, IL|
|North end||US 51 / WIS 29 in Rothschild, WI|
WI: Rock, Dane, Columbia, Marquette, Waushara, Portage, Marathon
Interstate 39 (I-39) is a highway in the midwestern United States. I-39 runs from Normal, Illinois at I-55 to Wisconsin Highway 29 (WIS 29) in Rib Mountain, Wisconsin, approximately six miles (9.7 km) southwest of Wausau.  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. I-39 was built in the 1980s and 1990s.
In Illinois, the route has a total length of 140.82 miles (226.63 km).  In Wisconsin, I-39 has a distance of 182 miles (293 km). With the exception of an eight-mile (13 km) segment around Portage, the Interstate shares a route with at least one other route number in I-39's entirety. From Rockford to Portage, I-39 is concurrent with I-90. 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 co-signed with the Interstate and has exit numbers based on its mileage.
In Illinois, I-39 begins at Interstate 55, north of the Bloomington-Normal, Illinois, area alongside of Route 251. It runs north largely through rural areas from the city of Normal. About 55 miles (89 km) north of the city, I-39 crosses the Illinois River over the Abraham Lincoln Memorial Bridge, which is 2,170.8 metres (1.3 mi) long.  Just north of the Illinois River, I-39 runs east of the cities of LaSalle and Peru before intersecting Interstate 80. 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 with I-88 near Rochelle. Further north, I-39 crosses the Kishwaukee River before meeting US 20 on the south side of Rockford. I-39 then runs east concurrently with US 20 to where the interstate joins the Jane Addams Memorial Tollway and Interstate 90 near Cherry Valley. I-39 and I-90 head north together to South Beloit. There is a toll plaza just south of Rockton Road. US 51 leaves I-39/90 at Illinois 75 in South Beloit, while I-39/90 continue north into Wisconsin.
For all but 1 mile (2 km) that Interstate 39 is in Illinois, it is designated concurrently with U.S. Route 51. The southern terminus of I-39 is less than 1 mile (2 km) from Interstate 74 as it runs around Normal.
I-39 enters from Illinois along with I-90, passing under Stateline Road (Hwy P), and bypasses Beloit to the east. East of the town, the route has a cloverleaf interchange that serves as the terminus for both WIS 81—which heads westward into Beloit—and I-43, which provides access to Milwaukee. I-39/I-90 has 3 interchanges (including one in Illinois) that serves Beloit. The I-39/90 concurrency continues to the north and is joined by WIS 11 about 7 mi (11 km) north of the I-43 interchange. The route bypasses Janesville to the east, although interchanges with US 14 and WIS 26 provide access to the town. There are 4 exits that serve Janesville. The route continues generally to the north, crossing the Rock River before having an interchange with WIS 59 that provides access to Edgerton to the west.  Subsequently, the route enters Dane County as it passes west of Lake Koshkonong. It is joined by US 51 from Edgerton and serves as the southern terminus of WIS 73. US 51 leaves the route 4 mi (6.4 km) to the north, about 7 mi (11 km) east of Stoughton. The Interstate gradually turns westward around Utica to an interchange with CTH N. It then turns gradually back to the north and interchanges with US 12 and US 18 in Madison. I-39 and I-90 bypass Madison to the east, and I-94 joins the concurrency at the eastern terminus of WIS 30, an interchange known as the Badger Interchange.  About 2 mi (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 (5 km) northwest of the US 151 interchange and WIS 19 another mile northwest of the US 51 interchange. Access is provided to CTH V 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. 
The Interstates cross WIS 60 at an interchange three miles (4.8 km) north of the county line west of Arlington 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 and I-94 and turns northward while the other two interstates turn northwest. 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 3 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 northeast to an interchange with US 51. The US route joins the Interstate and both turn north once again and leave the Portage area and, after four miles (6.4 km), enter Marquette County. 
WIS 23 joins I-39/US 51 northbound, 4 miles (6.4 km) from the county line. The three highways pass along Buffalo Lake and encounter a south-side half interchange with CTH D in the town of Packwaukee. WIS 23 leave 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.  Four miles north of the county line, I-39 / US 51 junction with WIS 21 in Coloma. I-39/US 51 meet 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.  In Portage County, I-39/US 51 takes a straight due north trajectory which provides access to CTH D, CTH W and WIS 54 (also Business US 51) over twelve miles (19 km). The WIS 54 interchange and the CTH B interchange a mile and a half north of it provide access for Plover. The next four interchanges—CTH HH, US 10 Eastbound / WIS 66 Westbound, Stanley Street and Business US 51 provide access to Stevens Point. Amongst these interchanges, the freeway turns northwestward, bypassing the city to the east and north. I-39/US 51 cross two more interchanges while in Portage County: Casimir Road four miles (6.4 km) northwest of Stevens Point,  and Westbound US 10 two miles (3 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. 
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 further north in Mosinee. Maple Ridge Road crosses after another two miles (3.2 km) as the freeway turns northeastward into Kronenwetter.  An interchange with Business 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. 
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".
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. 
Over the course of the 1970s, planning for the US 51 supplemental freeway (officially known as the North–South Freeway) took place in earnest. However, debate ensued over what type of highway should be built. IDOT wanted the entire highway built to Interstate standards, but a transportation committee established to review the proposed supplemental freeway system recommended only Interstate/freeway construction between Rockford and I-80.  The highway from Oglesby south to Decatur was recommended to be a four-lane divided expressway, utilizing the existing road where possible. After a decade of lobbying and back and forth debate, Governor James R. Thompson announced in 1986 that US 51 would be rebuilt to Interstate standards from Oglesby to Bloomington– Normal. 
However, due to funding concerns and intense local opposition, it was decided that US 51 would not built to Interstate standards between Bloomington and Decatur.  The Bloomington-Decatur segment was instead upgraded to a four-lane divided expressway, with mostly at-grade intersections, just as the transportation committee proposed. To this day, there remains ramp stubs northwest of the city along I-72, where the highways were to connect. 
The first segment of the freeway opened 1984 from IL 5 (now I-88) in Rochelle, to US 20 in Rockford.  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. 
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.  In December 1989, the section from Bloomington–Normal to Hudson opened, a distance of about 4.4 miles (7.1 km)  In early September 1992, another segment opened from IL 116 north to IL 17. 
In Wisconsin, the highway was officially designated in 1992.  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 Highway Department had to reconstruct the interchange connecting I-90 and I-94 with WIS 78 near Portage.  Signs denoting I-39 were placed along the highway in Wisconsin until 1996, when the section between Portage and Rib Mountain, Wisconsin (near Wausau) received its signs.  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.  The remaining segment along I-90/I-94 was not signed for I-39 until late 1998  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.  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. 
I-39 was considered at one point for the unbuilt Peoria-to-Chicago Highway.
The Illinois Department of Transportation is in the preliminary design stages for a project involving reconstruction of I-39 from US 20 east to Harrison Avenue in Rockford. The goal of the project is to reconstruct ramps to modern-day standards, reconstruct and widen I-39 to six lanes between the US 20 and I-90 interchanges, and allow southbound I-39 traffic to use through lanes at the US 20 interchange.  Currently, southbound I-39 traffic must exit from the US 20 freeway on a single lane ramp.
This section contains a table that is missing mileposts for one or more junctions.
|State||County||Location||mi ||km||Exit ||Destinations||Notes|
|Illinois||McLean||Normal||0.00||0.00||—||I-55 / US 51 south to I-74 – Decatur, Chicago, Peoria, Indianapolis, St. Louis||Southern end of US 51 concurrency; exit 164 on I-55|
US 51 Bus. – Bloomington, Normal
|||8.5||13.7||8||IL 251 (Lake Bloomington Road)|
|Woodford||El Paso||14.9||24.0||14||US 24 – El Paso, Peoria|
|Woodford||23.1||37.2||22||IL 116 – Peoria, Pontiac|
|Marshall||Wenona||35.7||57.5||35||IL 17 – Lacon, Wenona|
|LaSalle||Lostant||42.3||68.1||41||IL 18 – Henry, Streator|
|||52.4||84.3||51||IL 71 – Hennepin, Oglesby|
|||53.1||85.5||52||IL 251 – Peru, LaSalle|
|Illinois River||Abraham Lincoln Memorial Bridge|
|La Salle||57.5||92.5||57||US 6 – LaSalle, Peru, Ottawa|
|59||I-80 – Chicago, Des Moines, Moline, Rock Island||Signed as exits 59A (east) & 59B (west); exit 79 on I-80|
|Troy Grove||66.8||107.5||66||US 52 – Troy Grove|
|Mendota||72.7||117.0||72||US 34 – Mendota, Earlville|
|Lee||Paw Paw||83.2||133.9||82||Paw Paw|
|||88.1||141.8||87||US 30 – Sterling, Rock Falls, Aurora|
I-88 / IL 110 (CKC) ( Ronald Reagan Memorial Tollway) – Moline, Rock Island, Chicago
|Signed as exits 97A (east) & 97B (west). Exits 78A-B on I-88.|
|100.1||161.1||99||IL 38 – DeKalb, Rochelle|
|||104||IL 64 – Sycamore, Oregon|
|Monroe Center||111||IL 72 – Genoa, Byron|
|Rockford||US 20 west (Rockford Bypass) – Freeport, Rockford||Southern end of US 20 concurrency, I-39 southbound exits US 20 via exit 97|
|Cherry Valley||122A||US 20 east – Belvidere||Northern end of US 20 concurrency|
I-90 east ( Jane Addams Memorial Tollway) – Chicago
|Southern end of I-90 concurrency; I-39 uses I-90's exit numbers from here north|
US 20 Bus. (State Street)
|Last free exit northbound|
|12||Riverside Boulevard||Toll on northbound exit and southbound entrance|
|Machesney Park||9||IL 173 (West Lane Road)||Toll on northbound exit and southbound entrance|
|Rockton||South Beloit Toll Plaza|
|South Beloit||3||Rockton Road (CR 9)||Last free exit southbound|
|1||US 51 north / IL 75 – South Beloit||Northern end of US 51 concurrency|
|Illinois–Wisconsin state line|
|Wisconsin||Rock||Town of Turtle||2.48||3.99||185B||I-43 north – Milwaukee||I-43 exit 1|
|2.46||3.96||185A||WIS 81 west – Beloit|
|4.78||7.69||183||CTH-S (Shopiere Road)|
|Town of La Prairie||9.95||16.01||177||WIS 11 west – Janesville, Avalon||Eastern end of WIS 11 concurrency|
|Janesville||12.49||20.10||175||WIS 11 ( Bus. US 14) – Delavan||Western end of WIS 11 concurrency|
|15.61||25.12||171C||US 14 east|
|15.66||25.20||171B||US 14 west / WIS 26 south – Janesville|
|16.41||26.41||171A||WIS 26 north – Milton|
|Town of Fulton||24.62||39.62||163||WIS 59 – Edgerton, Milton|
|Dane||Town of Albion||27.60||44.42||160||US 51 south / WIS 73 / WIS 106 – Edgerton, Deerfield||Eastern end of US 51 concurrency|
|Town of Christiana||31.30||50.37||156||US 51 north – Stoughton||Western end of US 51 concurrency|
|Town of Pleasant Springs||40.40||65.02||147||CTH-N – Stoughton, Cottage Grove|
|Town of Blooming Grove||45.54||73.29||142||US 12 / US 18 – Madison, Cambridge||Signed as exits 142A (west) and 142B (east)|
|Madison||49.39||79.49||138B||WIS 30 west – Madison|
|Town of Burke||49.74||80.05||138A||I-94 east – Milwaukee||Eastern end of I-94 concurrency|
|51.74||83.27||135C||High Crossing Boulevard||Northbound exit and southbound entrance|
|Madison||51.96||83.62||135B||US 151 north – Sun Prairie|
|52.00||83.69||135A||US 151 south – Madison|
|Town of Burke||55.20||88.84||132||US 51 – Madison, De Forest|
|Community of Windsor||56.68||91.22||131||WIS 19 – Waunakee, Sun Prairie|
|Town of Vienna||60.93||98.06||126||CTH-V – Dane, De Forest|
|Columbia||Town of Arlington||67.97||109.39||119||WIS 60 – Lodi, Arlington|
|Town of Dekorra||71.99||115.86||115||CTH-CS – Poynette, Lake Wisconsin|
|Town of Caledonia||78.63||126.54||I-90 west / I-94 west – Wisconsin Dells||Northern end of I-90/I-94 concurrency; I-39 northbound exits I-90/94 via exit 108B|
|79.26||127.56||84||WIS 78 south – Merrimac||Northbound access is via I-90/94 exit 108A|
|79.73||128.31||85||Cascade Mountain Road|
|Portage||81.73||131.53||87||WIS 33 – Portage, Baraboo|
|84.13||135.39||89||WIS 16 to WIS 127 – Portage, Wisconsin Dells||Signed as exits 89A (east) and 89B (west) northbound|
|Town of Fort Winnebago||85.79||138.07||92||US 51 south – Portage||Southern end of US 51 concurrency; exit numbers from here north based on US 51's mileposts|
|Marquette||Town of Moundville||94.43||151.97||100||WIS 23 west / CTH-P – Wisconsin Dells, Endeavor||Southern end of WIS 23 concurrency|
|Town of Oxford||98.61||158.70||104||CTH-D – Packwaukee||Northbound exit and southbound entrance|
|100.77||162.17||106||WIS 23 east / WIS 82 west – Oxford, Montello||Northern end of WIS 23 concurrency|
|Westfield||107.32||172.71||113||CTH-E / CTH-J – Westfield|
|Waushara||Coloma||118.29||190.37||124||WIS 21 – Coloma, Necedah|
|Town of Hancock||125.35||201.73||131||CTH-V – Hancock|
|Town of Plainfield||130.94||210.73||136||WIS 73 – Plainfield, Wisconsin Rapids|
|Portage||Town of Pine Grove||134.07||215.76||139||CTH-D – Almond|
|137.56||221.38||143||CTH-W – Bancroft, Wisconsin Rapids|
|Town of Plover||145.91||234.82||151|
WIS 54 / Bus. US 51 – Wisconsin Rapids, Plover, Waupaca
|147.51||237.39||153||CTH-B – Wisconsin Rapids, Plover, Amherst|
|150.53||242.25||156||CTH-HH – Whiting|
|Stevens Point||152.71||245.76||158||US 10 east – Stevens Point, Waupaca, Appleton, Marshfield||Signed as exits 158A (east) and 158B (west) northbound; Southern end of US 10 concurrency|
|Town of Hull||153.94||247.74||159||WIS 66 – Stevens Point, Rosholt|
Bus. US 51 – Stevens Point
|Town of Hull||157.63||253.68||163||Casimir Road|
|159.75||257.09||165||US 10 west – Marshfield||Northern end of US 10 concurrency|
|165||CTH-X||Former diamond interchange; removed for construction of US 10 exit|
|Town of Dewey||165.39||266.17||171||CTH-DB – Knowlton, Lake DuBay|
|Marathon||Town of Knowlton||169.64||273.01||175||WIS 34 – Knowlton, Wisconsin Rapids|
|Mosinee||173.57||279.33||179||WIS 153 – Mosinee, Elderon|
|Town of Kronenwetter||175.39||282.26||181||Maple Ridge Road|
Bus. US 51 – Rothschild, Wausau, Schofield
WIS 29 east –
US 51 north / WIS 29 west – Wausau, Chippewa Falls
|Northern end of I-39 immediately before interchange; northern end of US 51 concurrency; freeway continues as US 51/WIS 29|
|1.000 mi = 1.609 km; 1.000 km = 0.621 mi|
- Federal Highway Administration (October 31, 2002). "Table 1: Main Routes of the Dwight D. Eisenhower National System of Interstate and Defense Highways as of October 31, 2002". Route Log and Finder List. Federal Highway Administration. Retrieved March 28, 2007.
- Google (February 15, 2008). "Overview Map of I-39" (Map). Google Maps. Google. Retrieved February 15, 2008.
- Illinois Technology Transfer Center (2006). "T2 GIS Data". Illinois Department of Transportation. Retrieved November 8, 2007.
- Federal Highway Administration. "Structure 000050019120847". National Bridge Inventory. Federal Highway Administration. Retrieved June 1, 2012.
- Wisconsin Department of Transportation (2013). Official State Highway Map (Map) (2013–2014 ed.). 1:823,680. Madison: Wisconsin Department of Transportation.
- Buschkopf, Jim. "I-94 reconstruction to be discussed September 19" (Press release). Wisconsin Department of Transportation. Archived from the original on August 27, 2008. Retrieved January 12, 2007.
- Google (January 12, 2008). "Casimir Road, Stevens Point, Wisconsin" (Map). Google Maps. Google. Retrieved January 12, 2008.
- Google (January 12, 2008). "Maple Ridge Road, Mosinee, Wisconsin" (Map). Google Maps. Google. Retrieved January 12, 2008.
- Priority Primary Routes (PDF) (Map). Illinois Department of Transportation. April 12, 1978. Retrieved March 23, 2010.
- Wills, Bill (August 27, 1976). "Sides Square Off in Hearings on Freeway". The Pantagraph. Bloomington, IL. p. 3. Retrieved March 7, 2015 – via Newspapers.com. (Subscription required (help)).
- Featherly, Marc (March 11, 1986). "Close Encounters on US 51 Trip". The Pantagraph. Bloomington, IL. p. 1. Retrieved March 6, 2015 – via Newspapers.com. (Subscription required (help)).
- "Politics Helped Shelve US 51 Work". The Pantagraph. Bloomington, IL. p. 34. Retrieved March 6, 2015 – via Newspapers.com. (Subscription required (help)).
- Google (March 6, 2015). "39.906955°N, 88.987777°W" (Map). Google Maps. Google. Retrieved March 6, 2015.
- "Interstate 39". Interstate Guide. AA Roads (Self-published). July 29, 2008. Retrieved January 11, 2009.[ unreliable source]
- 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.
- Carlson, Rich (November 18, 2007). "Routes 21 through 40". Illinois Highways Page. Self-published. Retrieved January 11, 2009.[ unreliable source]
- Zehr, Melissa (March 8, 1991). "Interstate 39: Crews Work to Link Highway by 1992". The Pantagraph. Bloomington, IL.[ full citation needed]
- Davis, Jenni (September 2, 1992). "I-39 Nearly Finished; 12 More Miles Open Today". Peoria Journal-Star – via Newsbank. (Registration required (help)).
- Bessert, Chris (January 2, 2008). "Highways 30-39". Wisconsin Highways. Self-published. Retrieved January 12, 2008.[ unreliable source]
- Walters, Steven (January 16, 1996). "US 51 will become I-39 between Portage, Wausau". Milwaukee Journal Sentinel. Retrieved January 12, 2008.
- Bessert, Chris (January 2, 2008). "Highways 70-79". Wisconsin Highways. Self-published. Retrieved January 12, 2008.[ unreliable source]
- Illinois Department of Transportation. "Interstate 39 / US 20 Improvement Project". Illinois Department of Transportation. Retrieved January 26, 2015.
- Wisconsin State Trunk Highway Logs:
- 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. Retrieved June 30, 2007.
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