- Southern Terminus:
- Montauk Highway (NY 27A) in Babylon
- Northern Terminus:
- Northern Parkway, Deer Park Road (Suffolk CR 35), and East Deer Park Road (Suffolk CR 66) in Dix Hills
- 9.05 miles
The Babylon-Northport Expressway was planned as a limited-access highway from Montauk Highway (NY 27A) in Babylon to Fort Salonga Road (NY 25A) in Northport. It was intended to carry the NY 231 designation for its entire length. The southernmost segment of the proposed expressway was constructed, beginning at Montauk Highway and ending just south of the Southern Parkway. North of this point, the NY 231 designation was assigned to Deer Park Avenue and Deer Park Road, which were formerly county roads. This designation was intended to be temporary. It would be removed after the remainder of the expressway was constructed. However, this never actually happened, so the temporary designation became permanent. Like the Seaford-Oyster Bay Expressway (NY 135), which does not go to Oyster Bay, the Babylon-Northport Expressway does not go to Northport. Just north of the interchange with the Northern Parkway in Dix Hills, Deer Park Road splits into Deer Park Road (CR 35) and East Deer Park Road (CR 66). The NY 231 designation ends at this point.