Interstate 90

Route information
Length: 3,021.22 mi (4,862.18 km)

Interstate 90 (I-90) is an east–west transcontinental freeway and the longest Interstate Highway in the United States at 3,021 miles (4,862 km). 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.

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History

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Non-tolled construction

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The western states were the last to complete their segments of I-90. Wyoming opened its final section, from the Montana state line to Sheridan, in July 1985 and dedicated it three months later following the completion of Montana's cross-border section. The last two-lane section in Montana, near Springdale, was upgraded to four lanes in May 1987. One of the last rural sections of I-90 to be built was through Wallace, Idaho, which placed its downtown on the National Register of Historic Places in 1976 to prevent its demolition for the freeway. The 1.5-mile (2.4 km) elevated freeway bypassed Wallace to the north and cost $42 million (equivalent to $76 million in 2021 dollars) to construct. It opened on September 12, 1991, and the city ceremonially retired the last stoplight on I-90. The Idaho section was declared fully complete in July 1992 after the Veterans Memorial Centennial Bridge opened near Coeur d'Alene.

Completion and later projects



The Homer M. Hadley (left) and Lacey V. Murrow (right) floating bridges carry I-90 across Lake Washington from Seattle to Mercer Island.

Washington was the last state to complete its section of I-90, primarily due to disputes and litigation over the Seattle–Bellevue section. The Snoqualmie Pass section was completed in 1981 with a viaduct for westbound traffic that stands 150 feet (46 m) over Denny Creek. The viaduct replaced an earlier plan for a ground-level freeway at the behest of environmentalists; the Mountains to Sound Greenway was established in 1990 along the corridor between Seattle and Thorp to preserve wilderness and recreational areas and was designated as a National Scenic Byway in 1998, a first for an Interstate Highway. The extension into Seattle was completed in stages between 1989 and 1993 and cost $1.56 billion (equivalent to $2.68 billion in 2021 dollars) to construct. The project involved construction of a new floating bridge, expansion of the Mount Baker Ridge Tunnel, addition of lids with parks, and extensive mitigation for environmental and social impacts. The project was originally planned to be completed in 1992, but was delayed a year due to the sinking of the original floating bridge during renovations in November 1990; the bridge was rebuilt and opened for eastbound traffic on September 12, 1993.

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