There have been five US 5A's in the state, all gone now. (There were even more instances of US 1A.)

"5W" and "5E" become 5 and 5A (or 5A and 5)

In the early 1930s, the state built "by-pass" or "cut-off" routes of US 5 for North Haven, Meriden, Berlin, and South Windsor. In 1931, the state proposed using "5W" and "5E" designations for the pairs of bypass and local routes. The American Association of State Highway Officials (AASHO) demurred, recommending the use of "A" for "Alternate" instead.

The state then assigned US 5A to the newer routes and left US 5 on the original routes. However, many motorists were reluctant to take the alternate routes, diluting their advantages.

To fix this, in 1939, the state notified AASHO of its intent to switch US 5 and US 5A: now US 5 would follow the new bypasses, and US 5A would follow the old US 5. For that reason, many instances of US 5 have two definitions, and the switch was made in 1940.

New Haven and North Haven

The original US 5A here, designated in 1932, was about 5.95 miles long, starting at the corner of State Street and Ferry Street in New Haven. US 5A continued north on State Street, then turned east on Broadway in North Haven to end at Washington Avenue, which was US 5 at the time. This alternate route allowed trucks to bypass two low railroad underpasses near the Quinnipiac River.

In 1940, US 5A became part of US 5 (which it is today). US 5A was moved to 6.11 miles of the original US 5, following Ferry Street, Middletown Avenue, Quinnipiac Avenue, Maple Avenue, and Church Street to end at Broadway in downtown North Haven.

US 5A was cancelled in parts here, in 1962, 1963 and 1966. The Middletown Avenue portion, an overlap with Route 17, became Route 17 by itself. The rest, starting with Quinnipiac Avenue, became SR 560 in 1962, then SR 750 in 1964. In 1987, SR 750 became part of Route 103.

Wallingford and Meriden

It may be hard to picture Broad Street in Meriden as a bypass route; in the 1930s, though, it was. US 5 followed Old Colony Road, Colony Street and North Colony Road from Wallingford though downtown Meriden into Berlin. US 5A followed Broad Street, and was about 6.04 miles long.

In 1940, US 5 and US 5A switched places; US 5A was now 6.97 miles long. In 1947, part of US 5A was turned over to the city of Meriden.

In 1962, US 5A was cancelled. From Hanover Avenue in Meriden south to US 5 became an extension of Route 71.

Berlin

The original US 5 in Berlin followed Worthington Ridge Road, now a local street to the west near where Route 9 crosses today. Before the Berlin Turnpike was built, a 1.75-mile bypass of Berlin to the east became US 5A.

In 1940, US 5 and US 5A switched places; US 5A was now 1.85 miles along Worthington Ridge Road. In 1942, US 5 was upgraded as part of the new four-lane divided Berlin Turnpike.

In 1962, US 5A was cancelled. The northern part became part of SR 572, as a connector from US 5 southbound to the new Route 72 freeway, and has that designation today.

In the application to AASHO to remove the routing, state highway officials commented that US 5A "should have been removed from the State system when the new U. S. 5 location was constructed years ago... [when I-91 is open to traffic], the U. S. 5A routing would be even more unnecessary."

South Windsor

Main Street in East Hartford and South Windsor was the original US 5. Circa 1940, the four-lane divided highway opened to the east; originally planned as US 5A, but quickly became US 5 itself. Main Street became US 5A.

For a few years there were two parallel US 5A routes, on either side of the Connecticut River. In 1945, US 5A on the South Windsor side was cancelled and Main Street was turned over to local maintenance.

Hartford and North (Jan. 1, 1932 to Oct. 24, 1968)

The most well-known US 5A was not a bypass, and not an old alignment of US 5. This route led from Hartford to Agawam, Mass. via Windsor Locks, and is now Route 159. The southern end was moved a few times; historical US 5A was usually a bit longer than Route 159 is today.

In 1929 or earlier, the route already existed in Massachusetts as state route 5A (in Connecticut, it was state highway 110). Connecticut originally wanted US 5W for the route, but AASHO countered with US 5A. It does not appear that Massachusetts ever converted its state 5A to a US route.

In 1942, US 5A extended 21.12 miles, from US 5 in Hartford to the state line. In 1944, possibly because US 5 had been relocated to the Charter Oak Bridge opening, US 5A now extended to the Berlin Turnpike in Wethersfield (using part of today's Route 314). This route followed Maple Avenue, Washington Street, Capitol Avenue, Trinity Avenue, High Street, and Main Street into Windsor. As the dike boulevard and North Meadows Expressway (today's I-91) were completed, US 5A was routed along those highways instead, then turning at today's Route 159 in Wilson.

By 1950, this US 5A existed in two parts: a 1.05-mile segment of Maple Avenue leading to the Hartford city line; and a 20.08-mile route starting at Pulaski Circle, then Trumbull, Jewel, Ford, and High Streets to Main Street.

In 1968, the state announced it was considering changing US 5A's designation to state route 159. In Massachusetts, the route would continue as MA 159. The reasoning:

• Since the new I-91 was the primary route north, US 5 was the alternate, and US 5A no longer merited a US route number
• The parallel US 5 and US 5A could lead to motorist confusion, especially along I-91, which crosses both routes multiple times

Now only one US alternate route exists in the state: US 1A in Stonington.