After John Street, the freeway crosses over the Long Island Rail Road's Montauk Branch, and thence continues northward through a large residential district.

[6] From its interchange with the Southern State Parkway, NY 231 continues northward as a four-lane boulevard named Deer Park Avenue.

The highway continues northward through Deer Park, crossing through a large commercial strip and underneath the Long Island Rail Road's Ronkonkoma Branch.

There is a short interchange with Long Island Avenue in Deer Park, from which point NY 231 continues north and into Dix Hills, where it once again becomes residential near the intersection with CR 2 (Straight Path).

[6] The arterial road remains residential, continuing northward to an interchange with Interstate 495 (the Long Island Expressway) in Dix Hills.

The road then continues northwards, soon reaching a partial cloverleaf interchange with the Northern State Parkway, where the NY 231 designation terminates.

[9] As suggested by its name, the Babylon–Northport Expressway was originally intended to be a north–south expressway spanning most of the width of Long Island, between NY 27A in the Incorporated Village of Babylon at its southern end and the intersection of Elwood Road, Fort Salonga Road (NY 25A), and Reservoir Avenue at its northern end, in the Incorporated Village of Northport.

[9] In 1962, the New York State Department of Public Works announced that Deer Park Avenue would be widened for a fourteen-mile stretch from Montauk Highway and NY 25A.