Interstate 90

Interstate 90 is an east–west transcontinental freeway and the longest Interstate Highway in the United States at. It begins in Seattle, Washington, and travels through the Pacific Northwest, Mountain West, Great Plains, Midwest, and the Northeast, ending in Boston, Massachusetts. The highway serves 13 states and has 15 auxiliary routes, primarily in major cities such as Chicago, Cleveland, Buffalo, and Rochester.
I-90 begins at Washington State Route 519 in Seattle and crosses the Cascade Range in Washington and the Rocky Mountains in Montana. It then traverses the northern Great Plains and travels southeast through Wisconsin and the Chicago area by following the southern shore of Lake Michigan. The freeway continues across Indiana and follows the shore of Lake Erie through Ohio and Pennsylvania to Buffalo. I-90 travels across New York by roughly following the historic Erie Canal and traverses Massachusetts, reaching its eastern terminus at Massachusetts Route 1A near Logan International Airport in Boston.
The freeway was established by the Federal-Aid Highway Act of 1956, replacing a series of existing U.S. Highways that had been preceded by local roads and auto trails established in the early 20th century. I-90 was numbered in 1957, reflecting its status as the northernmost transcontinental route of the system, and construction was underway on several sections with funding from the Federal-Aid Highway Act.
The route also incorporates several toll roads that predate the Interstate Highway System, including the Jane Addams Memorial Tollway, Indiana Toll Road, Ohio Turnpike, New York State Thruway, and the Massachusetts Turnpike. These toll roads opened in the 1950s and were followed by toll-free sections in Pennsylvania and Wisconsin that were finished in the 1960s. The Midwestern sections of I-90 were fully completed in 1978, and most of the route between Seattle and South Dakota opened by 1987. The final section, near the western terminus in Seattle, opened in September 1993; an eastern extension in Boston was completed in 2003 as part of the Big Dig project.

Route description

Washington

The western terminus of I-90 is at an intersection with Washington State Route 519 and 4th Avenue South in the SoDo neighborhood of Seattle. The junction is south of Downtown Seattle, adjacent to the Port of Seattle and two major sports stadiums, Lumen Field and T-Mobile Park. The freeway travels east through an interchange with I-5 and around Beacon Hill before it enters the Mount Baker Ridge Tunnel alongside the future 2 Line of the Link light rail system, set to open in 2026. I-90 emerges from the tunnel on a pair of floating bridges, among the longest of their kind: the eastbound-only Lacey V. Murrow Memorial Bridge and the Homer M. Hadley Memorial Bridge, which carries westbound traffic and the future light rail line.
The floating bridges cross Lake Washington to Mercer Island, where I-90 travels through a series of tunnels under of parkland, including Aubrey Davis Park. The freeway continues from the island and enters Bellevue, the largest city of the Eastside region, and intersects I-405 near Factoria. I-90 then travels along Lake Sammamish and through Issaquah as it leaves the Seattle metropolitan area and ascends into the Cascade Range on the Mountains to Sound Greenway, a designated National Heritage Area and National Scenic Byway. The freeway crosses Snoqualmie Pass, elevation, at the crest of the mountain range near a ski resort.
From Snoqualmie Pass, I-90 follows the Yakima River into the Kittitas Valley and intersects I-82 in Ellensburg after a brief concurrency with U.S. Route 97. The highway crosses the Columbia River on the Vantage Bridge and turns northeast to climb the cliffs of the Columbia Plateau near George. After traveling east across Moses Lake and the surrounding agricultural region, I-90 begins a long concurrency with US 395 at Ritzville as the highways turn northeast towards Spokane. I-90/US 395 is joined by US 2 through western Spokane, where it intersects US 195. The freeway crosses downtown Spokane on an elevated viaduct and splits from US 2 and US 395 to continue east across Spokane Valley towards the Idaho state line.

Idaho

I-90 traverses the Idaho Panhandle region at the north end of the state, where it connects Coeur d'Alene to communities in the Silver Valley. From the Washington state line, the freeway follows the Spokane River through Post Falls and Huetter to the city of Coeur d'Alene, where it intersects US 95, the state's main north–south highway. I-90 then turns southeast to bypass Coeur d'Alene and travel along a series of ridges that face Lake Coeur d'Alene, crossing an arm of the lake on the Veterans Memorial Centennial Bridge.
The freeway continues east across Fourth of July Summit and descends into the Silver Valley, where it follows the Coeur d'Alene River through several small towns along the historic Mullan Road. I-90 serves the cities of Kellogg and Wallace before it ascends into the Bitterroot Range and crosses Lookout Pass, which also marks the Montana state line.

Montana

Montana has the longest section of I-90, at almost, despite the highway only serving a portion of the state's east–west width. It descends from Lookout Pass along the St. Regis and Clark Fork rivers between the foothills of the Bitteroot Range and Coeur d'Alene Mountains. The freeway travels east through the Alberton Gorge and crosses the Clark Fork River several times before it reaches the head of the Missoula Valley. After a short concurrency with US 93, I-90 runs along the north side of Missoula and joins US 12 to continue southeast along the foothills of the Garnet Range and Sapphire Mountains.
After it splits from US 12 in Garrison, the freeway turns south to traverse the Deer Lodge Valley. It then turns east to serve Butte, where it overlaps with I-15 for and intersects I-115. I-90 then continues southeast and crosses the Rocky Mountains and Continental Divide at Homestake Pass, which is the highest point on the entire Interstate at. The freeway travels east across the Jefferson Valley and passes the headwaters of the Missouri River near Three Forks. It then enters the Gallatin Valley.
I-90 travels around Bozeman, where it is joined by US 191, and crosses Bozeman Pass between the Bridger and Gallatin mountains. At the east end of the mountains, the freeway begins to follow the Yellowstone River and is briefly concurrent with US 89, which serves Yellowstone National Park, and splits from US 191 at Big Timber. I-90 continues along the Yellowstone River through Billings, overlapping with US 87 and US 212, until it reaches Lockwood, the western terminus of I-94. The freeways split and I-90 continues east across the Bighorn Basin before it turns south near Hardin to follow the Little Bighorn River into the Crow Indian Reservation. The highway passes the site of the Battle of the Little Bighorn near Crow Agency and continues south along the river and the Wolf Mountains into Wyoming.
From 1995 to 1999, there was no numbered daytime speed limit on rural highways in Montana, including I-90. The speed limit was simply defined as "reasonable and proper" as determined on a case-by-case basis by the Montana Highway Patrol until the Montana Supreme Court ruled it was unconstitutional. The maximum daytime speed limit in Montana was initially set at in 1999 and was later raised to in 2015.

Wyoming

I-90 serves a portion of northeastern Wyoming that is primarily rural. The freeway, briefly concurrent to US 14, travels southeast along a series of creeks to Sheridan in the northeastern foothills of the Bighorn Mountains. I-90 and US 87 split in Sheridan and travel parallel to each other to Fort Phil Kearny, where they rejoin and continue south past Lake Desmet to Buffalo. The highways split again near Buffalo at a junction with I-25, which overlaps with US 87 to Casper.
From Buffalo, the highway turns east to cross the Powder River Basin, a region with several large coal mines. I-90 then reaches Gillette, where it begins a concurrency with US 14 and US 16 to a three-way split in Moorcroft. The freeway continues into the Bear Lodge Mountains and is rejoined in Sundance by US 14, which looped north to serve the Devils Tower. I-90/US 14 then continues northeast to Beulah, where it enters South Dakota.

South Dakota

I-90/US 14 enters South Dakota near Spearfish and travels east through prairie land, where it is briefly concurrent with US 85. Beyond Sturgis, the freeway turns south and follows the edge of the Black Hills to Rapid City, the gateway to Mount Rushmore. It then skirts the northern edge of Rapid City, which is served by spur route I-190, and passes Ellsworth Air Force Base while it continues east across the plains. I-90 splits from US 14 near Wall, home to the Wall Drug roadside attraction and located northeast of Badlands National Park.
The freeway travels southeast into the Buffalo Gap National Grassland and also passes a pair of decommissioned missile silos that form the Minuteman Missile National Historic Site. I-90 continues east along the top of a plateau that faces the White River and passes near Kadoka and Murdo. US 83 briefly joins the highway from Murdo to Vivian, where it splits off to serve the state capital of Pierre. It then crosses the Missouri River on the Lewis and Clark Memorial Bridge near Chamberlain and passes a rest area which overlooks the river and includes the Dignity statue. From Chamberlain, I-90 continues east across the plains and past several small towns near the city of Mitchell. It then reaches the Sioux Falls area, where it bypasses the city to the north and intersects I-29 and I-229. I-90 leaves Sioux Falls and crosses into Minnesota near Brandon.

Minnesota

I-90 crosses the southern portion of Minnesota and carries unsigned Legislative Route 391 across the state. From the South Dakota border near Beaver Creek to Albert Lea, the freeway travels east across farmland and towns in the plains and rolling hills of the Buffalo Ridge. It also intersects several north–south highways, including US 75 in Luverne, US 59 in Worthington, US 71 in Jackson, and US 169 in Blue Earth. I-90 travels around the northern outskirts of Albert Lea and intersects I-35 northeast of the city. It then reaches Austin and a brief concurrency with US 218.
From Austin, the freeway turns northeast to head towards Rochester, which it bypasses to the south and intersects US 63 and US 52. I-90 continues east into the hilly Driftless Area and descends from the bluffs that overlook Lake Onalaska on the Mississippi River. It turns southeast at Dakota and is joined by US 14 until the highways split near La Crescent. I-90 turns east before it reaches La Crescent, where it crosses the Mississippi River on the Dresbach Bridge into Wisconsin.