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 runs from Seattle, Washington, to Boston, Massachusetts, passing through the Pacific Northwest, Mountain West, Great Plains, and Midwest. The highway serves 13 states and has 15 auxiliary routes, primarily in major cities such as Chicago, Cleveland, Buffalo, and Rochester. The route incorporates several toll roads that predate the Interstate Highway System, including the Ohio Turnpike, New York State Thruway, and Massachusetts Turnpike. These toll roads opened in the 1950s and were followed by toll-free sections a decade later. The Midwestern sections of I-90 were fully completed in 1978, and the majority of the route between Seattle and South Dakota opened by 1987. The final section, near the western terminus in Seattle, opened on September 12, 1993; an eastern extension in Boston was completed in 2003 as part of the Big Dig project. (Full article...)