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. 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). 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. 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.

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. 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. 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. 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.

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.

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. 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. 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, 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.

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. 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.

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 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. 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. 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.

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 Department of Transportation (WisDOT) 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 (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.

State	County	Location	mi	km	Exit	Destinations	Notes
Illinois	McLean	Normal	0.00	0.00	—	I-55 / US 51 south to I-74 – Chicago, St. Louis, Decatur, Peoria, Champaign	Southern terminus of I-39; southern end of US 51 concurrency; I-55 exit 164; Central Illinois Regional Airport
			1.53	2.46	2	US 51 Bus. south (Main Street) – Bloomington, Normal
		Hudson	4.67	7.52	5	CR 12 (Franklin Street) – Hudson
		Hudson Township	7.75	12.47	8	IL 251 north / CR 8 (Lake Bloomington Road) – Kappa	Southern terminus of IL 251
	Woodford	El Paso	14.35	23.09	14	US 24 – Peoria, El Paso
		Woodford	22.37	36.00	22	IL 116 – Peoria, Pontiac
		Minonk	26.94	43.36	27	CR 2 – Minonk
	Marshall	Wenona	35.02	56.36	35	IL 17 – Lacon, Wenona
	LaSalle	Lostant	41.60	66.95	41	IL 18 – Henry, Streator
		Tonica	48.67	78.33	48	CR 54 (Reed Richardson Road) – Tonica
		Eden Township	51.70	83.20	51	IL 71 – Hennepin, Oglesby, Granville
			52.47	84.44	52	IL 251 – Peru, LaSalle
		Oglesby	53.79	86.57	54	Walnut Street – Oglesby
		Illinois River			Abraham Lincoln Memorial Bridge
		La Salle	56.75	91.33	57	US 6 (5th Street) – LaSalle, Peru, Ottawa
			58.68– 59.46	94.44– 95.69	59	I-80 – Chicago, Des Moines, LaSalle, Peru	Signed as exits 59A (east) and 59B (west); I-80 exit 79
		Troy Grove	66.16	106.47	66	US 52 – Troy Grove
		Mendota	72.09	116.02	72	US 34 – Mendota, Earlville
	Lee	Paw Paw	82.57	132.88	82	CR 10 (Chicago Road) – Paw Paw
		Willow Creek Township	87.34	140.56	87	US 30 – Sterling, Rock Falls, Aurora
		Steward	93.34	150.22	93	CR 2 (Perry Road) – Steward
	Ogle	Rochelle	97.10– 97.79	156.27– 157.38	97	I-88 Toll / IL 110 (CKC) (Ronald Reagan Memorial Tollway) – Moline, Rock Island, Chicago	Signed as exits 97A (east) and 97B (west); I-88 exit 78
			99.46	160.07	99	IL 38 / Lincoln Highway – DeKalb, Rochelle
		Lynnville Township	104.50	168.18	104	IL 64 – Sycamore, Oregon
		Monroe Center	110.98	178.60	111	IL 72 – Genoa, Byron, Kingston
	Winnebago	Rockford Township	115.49	185.86	115	CR 11 (Baxter Road)
		Rockford	119.05– 119.56	191.59– 192.41	—	US 20 west (Rockford Bypass) – Rockford, Freeport	Southern end of US 20 concurrency; Chicago Rockford International Airport
		Cherry Valley	122.17	196.61	122A	US 20 east – Belvidere	Northern end of US 20 concurrency
					122B	Harrison Avenue
			122.90	197.79	—	I-90 Toll east (Jane Addams Memorial Tollway) – Chicago	Southern end of I-90 concurrency; I-90 exit 17; I-39 uses I-90's exit numbers from here north
		Rockford	124.45	200.28	15	US 20 Bus. (State Street)	Last free exit northbound
			127.77	205.63	12	CR 55 west (East Riverside Boulevard)	Toll on northbound exit and southbound entrance
		Machesney Park	131.31	211.32	8	IL 173 (West Lane Road) – Machesney Park	Toll on northbound exit and southbound entrance
		Rockton	136.71	220.01	South Beloit Toll Plaza 1
		South Beloit	137.53	221.33	3	CR 9 (Rockton Road)	Last free exit southbound; northwestern end of Jane Addams Memorial Tollway
			139.93	225.20	1	US 51 north / IL 75 – South Beloit	Northern end of US 51 concurrency
			140.25 0.00	225.71 0.00	Illinois–Wisconsin state line
Wisconsin	Rock	Town of Turtle	2.48	3.99	185B	I-43 north – Milwaukee	I-43 exits 0A-B southbound; former cloverleaf interchange; undergoing conversion to tri-stack interchange.
			2.46	3.96	185A	WIS 81 west (Milwaukee Avenue) – Beloit	Former cloverleaf interchange; undergoing conversion to diverging diamond interchange for access to/from 43 via County route X.
			4.78	7.69	183	CTH-S (Shopiere Road)
		Town of La Prairie	9.95	16.01	177	WIS 11 west (Avalon Road) – Janesville, Avalon	Southern end of WIS 11 concurrency; Southern Wisconsin Regional Airport
		Janesville	12.49	20.10	175	Alt. I-39 north / Alt. I-43 north (Racine Street) / WIS 11 east – Delavan	Northern end of WIS 11 concurrency; former Bus. US 14
			15.61– 15.66	25.12– 25.20	171B	Alt. I-39 south / US 14 – Janesville	Formerly split into exits 171B (west) and 171C (east)
			16.41	26.41	171A	WIS 26 – Milton
		Town of Fulton	24.62	39.62	163	WIS 59 – Edgerton, Milton, Whitewater
	Dane	Town of Albion	27.60	44.42	160	US 51 south / WIS 73 / WIS 106 – Edgerton, Deerfield	Southern end of US 51 concurrency
		Town of Christiana	31.30	50.37	156	US 51 north – Stoughton	Northern 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 (Beltline Highway) – Madison, Cambridge	Signed as exits 142A (west) and 142B (east); US 12 exit 267
		Madison	49.39	79.49	138B	WIS 30 west – Madison	Left exit and entrance northbound; serves Dane County Airport; WIS 30 exit 3 eastbound and exit 240A westbound
			49.74	80.05	138A	I-94 east – Milwaukee	Left exit and entrance southbound; southern end of I-94 concurrency; I-94 exit 240
			51.74	83.27	135C	High Crossing Boulevard	Northbound exit and southbound entrance
			51.96– 52.00	83.62– 83.69	135B	US 151 north – Sun Prairie	Cloverleaf interchange
					135A	US 151 south – Madison
		Town of Burke	55.20	88.84	132	US 51 (Stoughton Road) – 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, Columbus
		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-90 exit 108B
			79.26	127.56	84	WIS 78 south – Merrimac	Signed as exit 108A northbound
			79.73	128.31	85	Cascade Mountain Road
		Portage	81.73	131.53	87	WIS 33 (Cook Street) – Portage, Baraboo
			84.13	135.39	89	WIS 16 (Wisconsin Street) to WIS 127 – Portage, Wisconsin Dells, Columbus	Signed as exits 89A (east) and 89B (west) northbound
		Town of Fort Winnebago	85.79	138.07	92	US 51 south (New Pinery Road) – 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, Princeton, Packwaukee	Northern end of WIS 23 concurrency
		Westfield	107.32	172.71	113	CTH-E / CTH-J – Westfield, Harrisville, Princeton
	Waushara	Coloma	118.29	190.37	124	WIS 21 – Coloma, Necedah, Wautoma, Oshkosh, Tomah
		Town of Hancock	125.35	201.73	131	CTH-V – Hancock
		Town of Plainfield	130.94	210.73	136	WIS 73 – Plainfield, Wisconsin Rapids, Wautoma, Nekoosa
	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 (Post Road) / Bus. US 51 – Wisconsin Rapids, Plover, Waupaca
		Village of Plover	147.51	237.39	153	CTH-B (Plover Road) – Wisconsin Rapids, Plover, Amherst
			150.53	242.25	156	CTH-HH (McDill Avenue) – Whiting, Stevens Point
		Stevens Point	152.71	245.76	158	US 10 east / WIS 66 west (Main Street) – Stevens Point, Waupaca, Appleton, Marshfield	Southern end of US 10 concurrency; southern end of WI 66 concurrency; signed as exits 158A (east) and 158B (west) northbound
			153.94	247.74	159	WIS 66 east (Stanley Street) – Stevens Point, Rosholt	Northern end of WI 66 concurrency
			155.76	250.67	161	Bus. US 51 (Division Street) – Stevens Point
		Hull	157.63	253.68	163	Casimir Road	To CTH-X
			159.75	257.09	165	US 10 west – Marshfield	Northern end of US 10 concurrency
						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 (Balsam Road) – Knowlton, Wisconsin Rapids
		Mosinee	173.57	279.33	179	WIS 153 – Mosinee, Elderon	Central Wisconsin Airport
		Kronenwetter	175.39	282.26	181	Maple Ridge Road
		Rothschild	179.52	288.91	185	Bus. US 51 – Rothschild, Kronenwetter
		Town of Rib Mountain	182.14	293.13	187	WIS 29 east – Weston, Green Bay US 51 north / WIS 29 west – Wausau, Chippewa Falls	Northern terminus; 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       Closed/former       Concurrency terminus       Electronic toll collection       Incomplete access

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