Route 18 is now West Street in Cromwell, part of Route 372. It was created in 1940, replacing SR 612, creating a 2.23-mile link between Route 72 (now Route 3) and Route 9 (now Route 99). In 1951, the state took down Route 18 signs, and the road became unsigned again: SR 525.
In 1963, the statewide road reclassification required all "secret routes" to have a number matching their districts. By coincidence, SR 525 fell in District 1 (secret routes in the 500s) and did not have to change its number.
On Dec. 21, 1989, a new segment of Route 9 opened in Berlin, triggering several route changes in the area. Route 372 was extended along old Route 72 into Cromwell, and replaced SR 525 to end at Route 99. The north-south part of Route 72 became an extension of Route 3.
For related info, see the Route 9 and Route 72 pages.
A side note: both the 1934 and 1938 biennial reports to the Highway Commissioner mention work on a "Route 18" in Colchester. A typo for Route 16? A new highway that never panned out? The one-liner mentions don't give enough information.