State route 91 was one of several routes that were renumbered because of conflicts with new Interstate routes designated in the 1950s. It followed an L-shaped, 23.58-mile route in eastern Connecticut:
- north along today's Route 198, from US 6 in Chaplin to Route 171 in Woodstock
- east along most of today's Route 171 to Peake Brook Road in Woodstock
- Peake Brook Road to Woodstock Avenue in Putnam, rejoining today's Route 171
- Woodstock Avenue (today's Route 171) to Providence Avenue
- probably (not proven) continuing Woodstock Avenue to possible old alignment of US 44 on Kennedy Drive
One or more alignment changed happened in Woodstock and Putnam:
- between 1945 and 1955, a new segment of Somers Turnpike and Woodstock Avenue opened, and Route 91 moved there from Peake Brook Road
- possible move in 1953 in downtown Putnam from Woodstock Avenue to Providence Avenue.
The evidence for the downtown move theory is in "Route Change Table.xls", provided by ConnDOT:
- in 1953, Route 91 was assigned "from Providence Avenue to US 44"
- In 1953, US 44 was moved from "several local roads" (not listed) in Putnam
- In 1954, US 44 was assigned to Grove Street (this is Route 12; doesn't make sense) and Front Street (now part of US 44)
Unfortunately for state Route 91, a new Interstate 91 was designated in 1957. State route 91 was decommissioned in 1959, and its mileage was assigned to Routes 171 and 198.