Route 122 officially begins as First Avenue in West Haven at the end of the southbound Exit 43 ramp of I-95 heading north.

First Avenue continues south of the interchange as an unsigned state highway (SR 745).

Forest Road travels for another 2.4 miles (3.9 km) following the line of the West River.

Route 158 was also established in 1932 and ran along Campbell Avenue and Forest Road from West Haven center (Campbell Avenue and Main Street) to Westville (Forest Road and Fountain Street).

After crossing the West River into New Haven, the road becomes Kimberly Avenue and has an interchange with I-95 at Exit 44.