Interstate 91 (I-91) is an Interstate Highway in the New England region of the United States. It is the primary north–south thoroughfare in the western part of the region. Its southern terminus is in New Haven, Connecticut, at I-95, while the northern terminus is in Derby Line, Vermont, at the Canada–United States border. Past the Derby Line–Rock Island Border Crossing, the road continues into Canada as Quebec Autoroute 55. I-91 is the longest of three Interstate highways whose entire route is located within the New England states and is also the only primary (two-digit) Interstate Highway in New England to intersect all five of the other highways that run through the region. The largest cities along its route, from south to north, are New Haven, Connecticut; Hartford, Connecticut; Springfield, Massachusetts; Northampton, Massachusetts; Greenfield, Massachusetts; Brattleboro, Vermont; White River Junction, Vermont; St. Johnsbury, Vermont; and Newport, Vermont.

New Haven is a city in New Haven County, Connecticut, United States. It is located on New Haven Harbor on the northern shore of Long Island Sound and is part of the New York City metropolitan area. With a population of 135,081 as determined by the 2020 U.S. census, New Haven is the third largest city in Connecticut after Bridgeport and Stamford, the largest city in the South Central Connecticut Planning Region, and the principal municipality of Greater New Haven, which had a total population of 864,835 in 2020. Prior to 1960, it was the county seat of New Haven County until the county governments were abolished that year.