- Interstate 90 (I-90) is a United States interstate highway that starts in Seattle, Washington and runs 3,021 mi (4,862 km) east through the states of Washington, Idaho, Montana, Wyoming, South Dakota, Minnesota, Wisconsin, Illinois, Indiana, Ohio, Pennsylvania, New York and Massachusetts in the East Coast in Boston. It serves as an important northern tier transcontinental highway across the Pacific Northwest, Rocky Mountains, Great Plains, the Midwest and the northeastern U.S. connecting the major cities of Seattle, Spokane, Coeur d’Alene, Missoula, Billings, Sheridan, Gillette, Sioux Falls, Albert Lea, La Crosse, Tomah, Madison, Wisconsin; Rockford, Elgin, Chicago, Sandusky, Cleveland, Erie, Buffalo, Rochester, Syracuse, Albany, Worcester and Boston; and the four state capitals of South Dakota (Pierre); Wisconsin (Madison), New York (Albany), and Massachusetts (Boston). It is the longest transcontinental interstate highway across the United States.