Route 27 was commissioned in 1949, as a 9.84-mile route leading from Route 84 (now Route 184) in Groton to Route 12 in Preston. It absorbed all of SR 677 and part of SR 664. Its route, from south to north:
- Col. Ledyard Highway, from Route 184 to Center Groton Road (taking over SR 677)
- Col. Ledyard Highway, continuing to Poquetanuck Road. This is now part of Route 117.
- Poquetanuck Road, west to Route 12. This is now part of Route 2A.
At the time, Route 27 intersected no other signed state highway between its endpoints. None of the original route exists as part of Route 27 today.
Today's Route 27 was originally called Route 169 (no relation to the modern route 169), and differed slightly from today's route: the southernmost leg followed Willow Street to US 1 instead of Dennison Avenue.
In 1951, Route 27 was extended south, including an overlap with Route 84, to incorporate all of Route 169. In 1960, Route 27 was modified to use today's alignment along Dennison Avenue.
In 1963, Route 27 was removed north of Route 184, leaving the 3.21-mile segment we see today. The Poquetanuck Road segment became SR 640, and later Route 2A. About half the Col. Ledyard Highway segment became part of Route 117. Some of the remainder became SR 641, which was turned over to the city of Groton in 1966.
Revival considered, for relief of Route 117
In 1970, the state considered reviving the section of Col. Ledyard Highway between Routes 184 and 117 as a state highway, as a temporary measure. The parallel section of Route 117 was narrow, curvy, and unsafe, and improvements would be expensive. The Ledyard Highway, though now a town road, was wider and straighter.
The plan was not adopted; instead, substantial improvements to Route 117 in Ledyard began in 1986.
Mystic attractions and traffic problems
Mystic Seaport, a re-creation of a 19th-century whaling village, opened in 1929. The Mystic Marinelife Aquarium opened in 1973. Even before the aquarium opened, planners were looking at solutions to growing traffic problems on Route 27.
In January 1973, the state briefly assigned a new route number – Route 319 – to Allyn Street, a parallel road intersecting I-95, to help relieve traffic on Route 27. Allyn Street was already a state road, but designated with the unposted "secret" number SR 614. The Route 319 announcement met with such opposition that the state rescinded the change ten days later.
In August 1973, as the state began a new study, the following alternatives were under discussion:
- new interchange on I-95, about a mile east, connecting to Jerry Browne Road and Coogan Boulevard
- shuttle buses between the attractions
- a monorail
- a new parallel road to Route 27
A new roadway, passing through residential areas, was proposed to carry northbound Route 27 traffic. Map shown in The Day
, New London, Conn.
In July 1974, the DOT and the I-95 and Rt. 27 Ad Hoc Advisory Committee released a plan for a new parallel road to the east, from Mistuxet Avenue to Coogan Boulevard, that would carry northbound Route 27 traffic. Existing Route 27 would carry southbound traffic. Within a week, Groton State Sen. Samuel Hellier, concerned about the plan's impact on residential areas, asked the state to halt the plan; his counter-proposals included moving the I-95 northbound exit ramp further north, and widening Route 27 in-place to 3 lanes.
In November 1974, the state announced there would be no major construction to address traffic problems.
Later, the state did make several small improvements in the area. The exit ramp from I-95 northbound widens to five lanes; Route 27 is four lanes for a short distance at the interchange; and the I-95 northbound entrance ramp was moved eastward to accommodate a new frontage road (SR 654) helping traffic get to the aquarium.