There have been three instances of Route 10A; all are gone now.
New Haven - Hamden, 1932 - 1970
This 4.61-mile route extended from US 5 in New Haven to Route 10 in Hamden, using Edwards Street and Whitney Avenue. This road was known in the late 1930s as being a better choice than Route 10 in the area.
It survived the 1961 statewide Route Reclassification, when many older routes were purged, but its southern end was truncated to the Hamden - New Haven city line, for a length of 3.62 miles. It remained a planned touchdown point for the proposed East Rock Connector from I-91 until that project was cancelled.
In 1970, Route 10A was demoted to unsigned SR 707, which exists today.
Cheshire, 1936 - 1962
This 1.20-mile route followed Main Street and Maple Avenue in Cheshire, bypassing Route 10 to the west. ConnDOT records show this route designated along a "new road" (probably Maple Street) in 1936. It was turned over to local maintenance on Aug. 1, 1962.
Plainville, 1932 - 1962
This 1.40-mile route following Whiting Street from Route 10 to E. Main Street (Route 72, now Route 372) in Plainville, and then overlapped Route 72 back to Route 10. It was removed from the state system in 1962.