It then runs through the Silvermine neighborhood near an interchange with Route 15 (Merritt Parkway) at the Norwalk-New Canaan town line.

Route 123 was commissioned in 1932 from the southern half of old Highway 184 (Norwalk to New Canaan) and a previously unnumbered road from there to the state line.

It originally began at US 1 and ran along Riverside Avenue on the west bank of the Norwalk River to New Canaan Avenue, then from there to the state line along modern Route 123.

The New Canaan Avenue bridge over the Norwalk River was a state road known as SR 404.

[2] The Route 123 bridge over the Norwalk River, which was undergoing repairs as of August 2007, was one of 12 bridges in the southwestern part of the state (including New Haven) with safety inspection ratings so low they are considered to be in critical condition.