Prior to the U.S. Highway system, the route was designated as Route 2 of the New England road marking system that existed between 1922 and 1927. When US 5 was first commissioned, it took over the entirety of Route 2. In 1927, US 5 had still not been paved in Vermont. Paving in Vermont was completed by 1933.
Connecticut
Originally, the route began at the New Haven Green in downtown New Haven. Between New Haven and North Haven, it originally ran east of the Quinnipiac River along modern-day Middletown Avenue and Route 103. In Hartford, US 5 passed by Central Row in the downtown area, entering the city via Maple Avenue and exiting on the Bulkeley Bridge with US 6. The southern terminus of US 5 has been realigned several times. By 1938, US 5 bypassed downtown New Haven via the Prospect Hill, Dixwell and West River neighborhoods, ending at Davenport Avenue and Columbus Avenue. In the mid-1950s, US 5 was instead routed through the Mill River neighborhood ending at East Street and Forbes Avenue. The modern New Haven to North Haven designation was also in place around this time. When I-91 opened in the 1967, the alignment was truncated to its current terminus at Exit 5 of I-91.
Massachusetts
The Massachusetts Highway Commission noted in 1912 that between Longmeadow and Agawam, the traffic between the Pioneer Valley and Connecticut was heavier on the east side. The western route from Hartford to West Springfield, via what is now River Road in Agawam and Route 159 in Connecticut, was designated US-5A. The main route, US-5, was originally designated to run on the east side of the river from Hartford into downtown Springfield, over the Memorial Bridge, along Memorial Avenue, up a four-lane Main Street, and then to Park Street, and up what is now Elm Street, to Riverdale Street. Riverdale Street at that time followed the northern part of the current Elm Street, and ended at Witch Path. In 1938, due to congestion in the West Springfield town center, Route 5 was moved to a newly constructed bypass from the North End Bridge to East Elm Street, where it continued north along the modern Elm Street and Riverdale Road. In 1941-42, a four-lane highway was built from East Elm Street to the current Elm Street fork just south of Morgan Road. The designations of both Riverdale Road and Route 5 were moved to the new more easterly route. In 1952-53, construction was started on the modern four-lane highway from the new South End Bridge in Agawam to the existing bypass at the North End Bridge. Tunnels were built under the rotaries at the Memorial and North End Bridges so Route 5 traffic and local traffic would not intersect. In 1971, the Riverdale Road portion of Route 5 was upgraded to a divided highway, with the installation of a steel median, and in 1987-88, a new lane was added to accommodate the new Riverdale Shops at Daggett Drive. The 1953 Massachusetts Department of Public WorksMaster Plan would have relocated U.S. 5 between Morgan Road in West Springfield and Mount Tom in Holyoke to a right-of-way similar to the current path of Interstate 91, but this was never implemented.
Vermont/New Hampshire
US 5 in Vermont mostly followed the New England Route 2 alignment when it was first designated. Minor deviations from the current route are found in the urban compacts of White River Junction, Burke Hollow, Coventry, and Derby Center. From 1927 to 1929, US 5 was temporarily routed into New Hampshire along part of what is now New Hampshire Route 12 between North Walpole and Charlestown. The US 5 designation was removed when the road opposite the Connecticut River in Vermont had been improved. In 1931, US 5 in Vermont was taken over by the state, which began paving the road. Over the years a few minor reroutings within populated centers have taken place. The current alignment was in place by 1975.
