New York State Route 231 (NY 231) is a north–south state highway located in Suffolk County, New York, in the United States. The route extends for from a partial interchange with NY 27A in Babylon to an interchange with the Northern State Parkway in Dix Hills. The southernmost of NY 231 is a limited-access highway known simply as Route 231; the remainder of NY 231 is known as Deer Park Avenue. Route description NY 231 begins at an interchange with NY 27A (Montauk Highway) in Babylon. The route progresses northward around Hawleys Lake as a four-lane freeway. A short distance after NY 27A, NY 231 meets ramps from County Route 50 (CR 50, named John Street), but only provides service to the county road from the southbound direction. After John Street, the expressway crosses over the Long Island Rail Road's Babylon Branch and crosses northward through a large residential district. After paralleling Madison Street in North Babylon, NY 231 intersects with NY 27 (the Sunrise Highway) in a full cloverleaf interchange. From NY 27, NY 231 heads northward and northwestward, intersecting with Hunter Avenue in North Babylon, where the expressway portion ends and the highway becomes an arterial boulevard. The route curves back to the north through the large commercial district, interchanging with CR 34 (Deer Park Avenue). Right after the interchange, NY 231 interchanges with the Southern State Parkway at a full cloverleaf in North Babylon. After the Southern State Parkway, NY 231 continues northward as an arterial boulevard named Deer Park Avenue. The surroundings are mostly commercial, continuing as a four-lane road and intersecting with CR 4 (Commack Road) at a fork. After Commack Road, NY 231 becomes a residential arterial with a grassy, divided median and intersecting with CR 57 (Bay Shore Road) in Deer Park. The highway continues northward through Deer Park, crossing through a large commercial strip and under another Long Island Rail Road line. There is a short interchange with Long Island Avenue in Deer Park as NY 231 continues into Dix Hills, where it becomes residential once again near the intersection with CR 2 (Straight Path). The arterial road remains residential, continuing northward into an interchange with Interstate 495 (the Long Island Expressway) in Dix Hills. Here, the road widens to five lanes and crosses underneath the interstate, where NY 231 becomes four lanes once again. The highway continues northward along the residential