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 changes 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)
Evidence of the Peake Brook Road move (which is not in doubt) exists on some maps, but might not be signed: a road named "Old Conn. 91", near the intersection of Harrisville Road and Woodstock Avenue in northwest Putnam (see the ConnDOT map linked below).
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.