In late 1930, Stonington officials passed a resolution to ask the state to incorporate Pequot Trail as a state road. This was even under consideration as a new "bypass" alignment of US 1, to avoid downtown Stonington and Mystic.

On Jan. 1932, Pequot Trail became a state road: part of the original State Route 84 (which became Route 184), leading from Groton to Pawcatuck via Old Mystic. In 1935, a new highway (now the Providence - New London Turnpike) opened from Old Mystic northeasterly to the Rhode Island state line. Route 84 was moved there, and Pequot Trail was once again a town road.

In 1940, part of the road became state-maintained SR 451; a 1958 town map shows this as a tiny section of Pequot Trail near Wheeler Drive.

In 1960, the state Committee to Reclassify All Public Roads recommended adding Pequot Trail as a secondary state highway. In 1963, it was added as unsigned SR 645. In 1964, when I-95 opened, the state relocated SR 645 slightly near Exit 91 (The diagonal Old Pequot Trail is the old alignment).

On Sept. 1, 1987, SR 645 was promoted to signed Route 234, and remains one of Connecticut's newest signed route numbers.