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Route 166 is the shortest signed state highway to cross a town line. The state did want to get rid of it in the early 1960s, but lost an arbitration dispute with the towns.
It's known as Spencer Plains Road throughout... but also (on I-95 signs and some business addresses) as the singular Spencer Plain Road. CTDOT town maps, Google Maps, and the town's own GIS use the plural.
In the 1920s, State Highway 166 followed today's Route 30 between Route 74 in Tolland and Route 190 in Stafford.
Route 166 was commissioned in 1935. Previously, but after 1932, the segment in Westbrook, from Route 153 to the Old Saybrook town line, had been added to the state system as SR 618.
In 1962, during the state highway reclassification effort, Route 166 was proposed to be decommissioned and turned over to the towns. However, local officials persuaded the state to keep the route. In 1965, its retention was official.