In 1922, when route numbering debuted in Connecticut, much of US 5 was marked New England Interstate route NE-2. In 1926, US 5 was created, leading from New Haven to downtown Hartford, across the Bulkeley Bridge, and north through Enfield into Massachusetts.
An alternate route north from Hartford, US 5A, was created on the west side of the Connecticut River. That's now Route 159.
In the 1930s, the State Street portion of US 5 in North Haven and New Haven was called US 5A; at the time, US 5 instead followed Maple Ave. (today's Route 103) and Middletown Ave. (today's Route 17) into New Haven.
In the 1940s, US 5 took a route around downtown New Haven to meet US 1 near the West Haven line. From State Street, US 5 followed Edwards Street and Henry Street to Sherman Avenue; then south on Sherman to Winthrop Avenue; then southwest on Davenport Avenue to intersect US 1 where Route 10 does now. Edwards and Henry Streets were later a part of Route 80, but they are locally maintained now.
In South Windsor, US 5 first used Main Street, until a four-lane divided section was built to the east in 1942. Main Street became US 5A for a few years, but is locally maintained now. In Enfield, a short section of King Street was upgraded to four lanes divided at about the same time. (its old alignment is Old King Street).
Also in 1942, the Berlin Turnpike was being upgraded to a four-lane divided highway.
In Hartford, US 5 originally passed through downtown, entering on Maple Street, and then crossing into East Hartford on the only bridge in the area, the Bulkeley. In 1942 the Charter Oak Bridge opened, and still carries US 5 today. Late 1945 saw the opening of the 4.5-mile South Meadows Expressway (now part of US 5/ Route 15), which had been delayed by World War II restrictions.
Relocated US 5 and Interstate 284
In 1963, the state unveiled a plan to relocated US 5 as a four-lane expressway from I-84 in East Hartford to I-91 in East Windsor. This was never implemented, although the Governor Street ramps in East Hartford (SR 500) would have been the southern terminus. In the 1970s, this proposal was called Interstate 284; see the I-284 page for more details.