The original Route 184, commissioned in 1932, was a 7.25-mile route in Bloomfield: starting at US 44, it followed Blue Hills Avenue (now Route 187), Park Avenue and Mountain Avenue (most of which is now Route 178) and West Street to end at Route 185. In 1963, this Route 184 was decommissioned, and Routes 187 and 178 got some of the pieces.
Most of the modern Route 184 started in the 1920s as State Highway 311. This route followed present-day 184 from the Thames River into Stonington, and present-day Route 201 and Mystic Road (SR 627) to Route 2 in North Stonington.
In 1932, SH 311 was decommissioned and state route 84 was born; instead of going northeast from Old Mystic, Route 84 instead continued east along present-day Route 234 to Pawcatuck. Some maps also show Route 84 following the old SH 311/new Route 201 to Route 2 for a few years.
Even in the 1920s and '30s, the need was seen for bypass routes to relieve the congested shore route, US 1. Bypasses to the west were built as Route 80 and the Merritt Parkway. East of the Thames River, this role was given to Route 84.
In 1936, a new road opened east-northeast from old Mystic to the state line, and Route 84 was rerouted there. (This date is from the 1938 WPA book "Connecticut: A Guide to its Roads, Lore, and People"; however, the 1934 official state map already showed Route 84 in its new configuration.) Route 84 was soon billed as a boring but functional inland alternative to crowded US 1. Also in 1936, the Route 201/SR 627 segment became Route 119.
In 1947, Connecticut proposed rerouting US 1 between New London and Providence to CT 84 (now Route 184) and RI 3 -- a more direct route between the cities. Rhode Island declined: it didn't want to remove US 1 from its shore towns, and lose money for four-laning the road there.
Rhode Island countered with a US 1A proposal for the 84/3 route. Connecticut agreed, and it was sent to AASHO. Afterward, it was either rejected or retracted, because US 1A was never posted along the route.
A 1953 State plan called for Route 84 to become a freeway; however, the freeway became Interstate 95 instead.
In 1958, Route 84 was renumbered CT 95, for number compatibility with the completed sections of I-95 elsewhere; in 1964, when I-95 opened in Groton and Stonington, state route 95 was renumbered 184, which it remains to this day. One of the first Hartford Courant articles to mention Route 184 by name discusses the adverse effect of the opening of I-95 on some Route 184 businesses dependent on drive-by business.
"Route 184 Extension"
Present-day Route 184 ends at Route 216, but the original Route 84 continued east into Rhode Island, becoming RI 3. Today, a road still continues into Rhode Island, but ends shortly after crossing a 1930s concrete bridge just east of the state line. On the Connecticut side, it's called the Providence - New London Turnpike (and SR 626). On the Rhode Island side, it's called Route 184 Extension. There's no public access from this road to the rest of Rhode Island without going back into Connecticut.
Circa 2003, Route 184's interchange with Route 12 was reconstructed. The ramps were realigned, and vertical clearance was increased 12'-2" to 16'-3", but the overall geometry of the 5-ramp interchange was unchanged.