Route 52 is the former number for Interstate 395 in Connecticut and Massachusetts. Both states applied for an interstate designation, which was approved in 1983. However, I-395 was not the first interstate number considered for the route.
The Route 52 number was applied Sept. 15, 1964 to two sections of highway that had not previously been numbered: a segment of the Connecticut Turnpike from Waterford to the US 6 cutoff (SR 695), and a segment of "relocated Route 12" extending north from SR 695 to Route 101. "Relocated Route 12" was more of a planning number for the non-turnpike section, since there isn't evidence of it having been signed Route 12.
Route 52 becomes an Interstate
The idea of designating Route 52 as an interstate highway dates to the early 1970s. What spurred this idea forward in Connecticut, however, was Rhode Island's cancellation of its portion of I-84 in 1982. Connecticut decided to still extend I-84 anyway, to end at Route 52 in Killingly. If Route 52 were another interstate highway, state officials reasoned that getting federal approval and funding for I-84 would be easier.
At the same time, in August 1982, Congressman Sam Gejdenson proposed renumbering Route 52 as an extension of I-290, which already existed in Massachusetts, for reasons of economic development .
In late 1982, Gov. William O'Neill asked the U.S. Transportation Secretary to consider an interstate designation, calling the route a "logical addition" as it links I-90 and I-95. Route 52 already met the physical standards of an interstate highway — the other main criterion for acceptance.
Shortly before this was granted, the decision was made to use the I-395 designation instead. A Hartford Courant article using the I-290 designation appeared just six days before the state announced, on June 20, 1983, that Route 52 was promoted to I-395.
Route 52/I-395 plays a part in the aftermath of the cancelled I-84 to Providence, plans to address tourist and casino traffic, and even the southern terminus of the long-awaited Route 11. There's more information at the I-395 page.