The maximum speed limit in Montana is 80 mph.

I-90 is a transcontinental highway that connects Seattle to Boston.The portion of the state of Montana goes through fourteen counties in central and southern Montana.It is the largest segment of I-90 within a single state due to the large size of Montana.

I-90 enters Montana and Mineral County from Shoshone County, Idaho over the 4,725-foot (1,450 m) high Lookout Pass, and immediately has its first interchange.The highway goes through the woods before meeting the rest area east of the Idaho–Montana border.There is a small road that goes into the St. Joe National Forest from the rest stop.

Saltese is a small unincorporated community that is served by another diamond interchange.The diamond interchange serving the unincorporated community of Haugan is served by each direction diverging for a small distance before returning side to side.[3][4]

Two miles after Haugan, I-90 intersects the census-designated place (CDP) of De Borgia, continuing southeasterly, intersecting several minor roads, before reaching the CDP of St. Regis and Montana Highway 135.The highway crosses over the Clark Fork River several times.Before the highway turns south through the Quartz Flats rest area, Superior is the county seat.The highway goes easterly after leaving the rest area.Just south of Alberton is where the highway exits the county.[3][4]

The highway crosses Alberton before going easterly through Huson and MT 263.The highway goes southeast before intersecting the highways U.S. Route 93 and MT 200.At the first exit of the three highways, US 93 leaves on the western border of Missoula and goes through a residential area.The last exit leads into East Missoula.US 12 joins I-90 and the other exits to the north of the highway outside of Missoula, paralleling the Blackfoot River.The Bearmouth rest area is from the Idaho border to the Missoula–Granite County border.2, 3, and 4

After entering Granite County, I-90 has an interchange with an access road that leads to the Garnet Back Country Byway.The highway goes north of the Lolo National Forest.The highway goes through northern Granite county before entering Powell County.The eastern segment of the Lolo National Forest is cut off by the intersection of I-90 and MT 272.As US 12 ends its concurrency, Garrison is northeast of the highway, traveling north from I-90 towards the state capital of Helena.The former Northern Pacific Railway trackage is parallel to I-90 before intersecting S-275 and Deer Lodge.I-90 exits Powell County at Race Track.[3][4]

I-90 splits the northern tip of Deer Lodge County.The highway goes west of the Warm Springs State Wildlife Management Area.After passing the wildlife management area, the highway goes by Warm Springs.The Deer Lodge–Silver Bow County border is just northwest of I-90.[3][4]

As the highway starts to turn back towards the east, the first exit in Silver Bow County is Fairmont Hot Springs.In the southwest corner of the interchange, I-15 becomes concurrent with I-90.I-115 goes east into town as I-15/90 goes southeast, circumventing most of downtown Butte.I-15/90 splits just east of Butte and goes north over the Continental Divide until it crosses the 6,368-foot (1,941 m) high Elk Park Pass, which straddles the Silver Bow–Jefferson County border.I-90 crosses over the divide at Homestake Pass to enter Jefferson County.An emergency escape ramp can be found on the highway's east side.Before meeting the Jefferson–Broadwater County border, Jefferson County has interchanges with S- 399 to the east of Pipestone, Montana, and S 358 west of Cardwell.[3][4]

There is only one exit along I-90 in Broadwater County that leads to the Missouri River, and it is located in the south of the highway.The towns of Manhattan and Belgrade are bypassed by I-90 on the north side of the highway.There is a new interchange just east of Belgrade that connects the city of Bozeman with the airport.Access to Bozeman Hot Springs can be found on the south side of I-90.The highway enters the city of Bozeman, the county seat, and intersects with US 191.The highest point in the county is the 5,760-foot high Bozeman Pass.The county seat of Livingston is where US 89 intersects I-90.The highway goes northeasterly, paralleling the river, before crossing the Park–Sweet Grass County border.[3][4]

The county seat of Big Timber is in the northwest corner of I-90.The Greycliff rest area is located 380.9 miles from the Idaho border.Just west of Reed Point, the highway passes through the Sweet Grass–Stillwater County border.I-90 crosses over the river to get to the county seat of Columbus, which is on the north side of the highway.The highway is named after Robert E. Ewing Jr.The Columbus rest area is 418.6 miles from the Idaho border to Park City.[3][4]

After entering the county seat ofBillings, US 212 becomes concurrent with I-90.I-90 bus.As the main freeway travels south of the city, it passes into town and leads to the intersection of MT 3 and US 87.Three highways intersect the western end of I-94, which leads northeast towards North Dakota, while I-90 leads southeast into Big Horn County.The seasonal Hardin rest area can be found immediately after entering Big Horn County.The three highways intersect at the county seat of Hardin.Access to Yellowtail Dam is provided by S-313.I-90 enters the Crow Indian Reservation and intersects S-384 before reaching Crow Agency.The Little Bighorn Battlefield National Monument and Reno–Benteen Battlefield Memorial are both located in Crow Agency.After leaving the state of Montana, I-90/US 87 passes by Lodge Grass and Wyola before entering Wyoming.[3][4]

Improvements to the I-90 corridor include the construction of new east and west bridges over the Yellowstone River, with each bridge having three to four traffic lanes.Reconstruction and widening of many of the bridges, interchanges and on-off ramps along the corridor at an estimated cost of $114 million is recommended.[8]

I-90 replaced US 10 between Livingston and the Idaho border.The section over Homestake Pass cost $18.6 million to build and was finished in 1966.The last two-lane section of the highway was expanded to four lanes in 1987.[13]

I-90 was not a divided highway until 1995 when it was near the Idaho border.

The daytime maximum speed limit in Montana was reasonable and prudent from 1995 to 1999.The daytime and nighttime speed limits on interstate highways are 80 miles per hour for cars and light trucks and 65 for heavy trucks, except in urban areas where the maximum speed limit is 65 mph.It was 13 and 14.