Interstate 90 in Montana

Interstate 90 (I-90) is an east–west transcontinental Interstate Highway across the northern United States, linking Seattle to Boston. The portion in the state of Montana is 552.54 mi in length, passing through 14 counties in central and southern Montana. It is the longest segment of I-90 within a single state.

Mineral County
I-90 enters Montana and Mineral County from Shoshone County, Idaho over the 4725 ft high Lookout Pass, which traverses the Coeur d'Alene Mountains of the Bitterroot Range, and immediately has its first interchange, a partial cloverleaf interchange serving extreme northwest Mineral County and access to Lookout Pass Ski and Recreation Area. The highway continues southeasterly through woodlands, paralleling the St. Regis River, before meeting the Dena Mora (Lookout Pass) rest area 4.7 mi east of the Idaho–Montana border. About 0.5 mi from the rest stop is a diamond interchange, serving a small minor road into the St. Joe National Forest in Idaho.

The highway continues, winding through the woodlands until another diamond interchange, serving the small unincorporated community of Saltese. Continuing east for about 6 mi, each direction diverges for a small distance before returning side to side for a diamond interchange serving the unincorporated community of Haugan.

2 mi 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 (MT 135). The highway stops paralleling the St. Regis River, and begins to parallel, and cross over several times, the Clark Fork River. Superior, the county seat, is intersected before the highway turns south through the Quartz Flats rest area, located 58.2 mi from the Idaho border. After leaving the rest area, the highway straightens out, heading easterly. The highway continues east before exiting the county, just south of Alberton.

Missoula, Granite, and Powell counties
Entering Missoula County, the highway intersects Alberton, before continuing easterly through Huson and Secondary Highway 263 (S-263). The highway continues southeast before intersecting the concurrent highways US Highway 93 (US 93) and MT 200, which join I-90 from the west. The three highways continue into the county seat of Missoula, where, at the first exit, US 93 leaves on the western border of Missoula and travels through Orchard Homes. The next two exits travel into Downtown Missoula, while the last exit serves East Missoula. US 12 joins I-90 and MT 200 exits to the north of the highway outside of Missoula, paralleling the Blackfoot River as I-90 continues east, following and then intersecting S-210 in Clinton. The Bearmouth rest area, 142.8 mi from the Idaho border, is intersected before crossing the Missoula–Granite county border.

After entering Granite County, I-90 has an interchange with an access road to the Garnet Back Country Byway, providing access to the Garnet Range on the west slope of the Rocky Mountains. The highway passes north of Lolo National Forest while headed easterly. The highway continues through northern Granite county, intersecting MT 1 before exiting the county entering Powell County. I-90 intersects S-272 13 mi after entering the county, which clips the eastern segment of Lolo National Forest, before the seasonal Gold Creek rest area is intersected. Garrison is northeast of the highway as US 12 ends its concurrency, traveling north from I-90 toward the state capital of Helena. I-90 turns south, paralleling former Northern Pacific Railway trackage, before intersecting S-275 and the county seat, Deer Lodge. Continuing south, I-90 exits Powell County at Racetrack.

Deer Lodge, Silver Bow, and Jefferson counties
I-90 bisects the northern tip of Deer Lodge County, in a southwesterly direction. Galen and S-273 are intersected, before the highway passes west of the Warm Springs State Wildlife Management Area. After passing the wildlife management area, the highway passes by Warm Springs and MT 48. Just northwest of the Deer Lodge–Silver Bow county border, I-90 intersects MT 1.

Fairmont Hot Springs Resort, accessible via S-441, is the first exit in Silver Bow County on I-90, as the highway starts to turn back toward the east. Ramsay is located in the southwest corner of the interchange where I-15 becomes concurrent with I-90 through Butte. On the western edge of Butte, I-115 continues east into town, as I-15/I-90 turns southeast, bypassing most of downtown Butte. An interchange with MT 2 serves Bert Mooney Airport before I-15/I-90 split just east of Butte, with I-15 continuing north over the Continental Divide over the 6368 ft high Elk Park Pass, which straddles the Silver Bow–Jefferson county border. I-90 heads south then east, also entering Jefferson County, passing over the divide at Homestake Pass which is 6375 ft high. After traveling the pass, an emergency escape ramp can be found on the highway's eastbound lanes. Jefferson County has interchanges with S-399 east of Pipestone, Montana and S-359 east of Cardwell before meeting the Jefferson–Broadwater county border.

Broadwater, Gallatin, and Park counties
There is only one exit along I-90 in Broadwater County, linking MT 2 in the south and US 287 north of the highway before crossing the Broadwater–Gallatin county border at the Missouri River. I-90 parallels S-205, bypassing the towns of Trident, Logan, Manhattan, and Belgrade on the north of the highway. A new interchange was constructed just east of Belgrade (exit 299) that services Bozeman Yellowstone International Airport to the north and the western area of the city of Bozeman. MT 85 travels south from I-90 in Belgrade, providing access to Bozeman Hot Springs. The highway then enters the city of Bozeman, the county seat, and intersects and becomes concurrent with US 191. I-90 exits the county at the Gallatin–Park county border at 5760 ft high Bozeman Pass. US 89 intersects I-90 in the county seat of Livingston and passes over the Yellowstone River. The highway continues northeasterly, paralleling the river, intersecting S-295 before crossing the Park–Sweet Grass county border.

Sweet Grass and Stillwater counties
Still paralleling the Yellowstone River, I-90 travels northwest intersecting S-298 and US 191 in the county seat of Big Timber. The highway turns back southeast, passing south of Greycliff and the Greycliff rest area, located 380.9 mi from the Idaho border. The highway passes through the Sweet Grass–Stillwater county border just west of Reed Point. After passing into Stillwater County, I-90 crosses over the river, and enters the county seat of Columbus, intersecting MT 78 south of the highway and S-306 on the north side of the highway. The highway is titled the Robert E. Ewing Jr. Memorial Highway between mileposts 410 and 424. I-90 intersects the Columbus rest area, 418.8 mi from the Idaho border, followed by Park City before exiting Stillwater County.

Yellowstone and Big Horn counties
US 212 becomes concurrent with I-90 just east of Laurel before entering the county seat of Billings. I-90 Business (I-90 Bus.) passes into town as the main freeway travels south of Billings and bisects Billings Heights and Lockwood intersecting MT 3 and US 87, which joins the I-90/US 212 concurrency. On the extreme eastern end of Billings, the three highways intersect the western terminus of I-94, which heads northeast toward North Dakota, while I-90 turns southeast into Big Horn County. Immediately after entering Big Horn County, the three highways pass the seasonal Hardin rest area. Entering Hardin, the county seat, the three highways intersect S-313 and MT 47. S-313 provides access to Yellowtail Dam, while MT 47 connects back to I-94. After exiting Hardin, I-90 turns south, entering the Crow Indian Reservation and intersects S-384, before reaching Crow Agency. Crow Agency is home to Little Bighorn Battlefield National Monument and Reno–Benteen Battlefield Memorial, which are passed after US 212 departs to the east. Continuing south, I-90/US 87 parallels S-451 and the Little Bighorn River, passing by Garryowen, Benteen, Lodge Grass, and Wyola before exiting the state of Montana and entering Sheridan County, Wyoming.

History
I-90 replaced US 10 between Livingston and the Idaho border, and, before that, the Yellowstone Trail. Some sections of the highway follow the historic Mullan Road, a military road constructed from 1859 to 1862 between Fort Walla Walla in modern-day Washington and Fort Benton in Montana.

The section over Homestake Pass cost $18.5 million (equivalent to $ in ) to construct and was completed on October 30, 1966. The final two-lane section of the highway, near Springdale, was expanded to four lanes in May 1987.

Until 1995 in Montana near the Idaho border, I-90 was not a divided highway for a few stretches, having only a narrow paved median.

From 1995 until 1999, the daytime maximum speed limit in Montana was "reasonable and prudent". , the daytime and nighttime speed limits on Interstate Highways are 80 mph for cars and light trucks and 65 mph for heavy trucks, except within urban areas, where the maximum speed limit is 65 mph for all vehicles, day or night. The western 34 mi have a lower speed limit, as well as the Missoula and Butte urban areas.

In 1996, a section of I-90 was closed for 19 days due to a train derailment and chlorine spill near Alberton.

Expansion of the Billings area corridor
The 2012 Billings area I-90 corridor planning study recommends many improvements to the corridor from Laurel through Lockwood, including construction of new east and westbound bridges over the Yellowstone River, with each bridge having three to four traffic lanes. Also recommended are construction of additional eastbound and westbound traffic lanes from Shiloh to Johnson Lane and reconstruction and widening of many of the bridges, interchanges and on/offramps along the corridor at an estimated cost of $114 million.