Another one of Westchester's unbuilt highways, the Ossining-South Salem Expressway was proposed in 1956 by the Westchester County Planning Department. Since the expressway was to utilize the NY 172 alignment, the expressway would probably have been given the NY 172 designation.
The planning department stated the reasons for constructing the proposed 18.7-mile, four-lane expressway as follows:
The purposes of this expressway are to provide adequate access across the wide central part of the county, and to permit traffic entering Westchester from the Danbury and Ridgefield areas of Connecticut destined for southerly points to connect with the various north-south routes.
The first 6.7-mile section of the Ossining-South Salem Expressway, which was designated as a high priority by the county, would was to begin at an interchange with the proposed US 9 expressway northeast of the Village of Ossining, in the vicinity of Maryknoll Seminary, and proceed east into the Town of New Castle on a route parallel to the existing NY 9A (Briarcliff-Peekskill Parkway). Departing the NY 9A alignment, the NY 172 expressway would have continued east to cross the Taconic State Parkway at the NY 100-NY 133 interchange near Echo Lake State Park. An interchange would have been provided at the Taconic State Parkway. Traversing open country at this point, the route would have continued northeast toward the Village of Mount Kisco. Near the Mount Kisco Country Club, the proposed route would have crossed the Saw Mill River Parkway and the MTA Metro North Harlem Line. Between the Saw Mill River Parkway and NY 117 (Bedford Road), there would have been an interchange with the proposed Central Corridor Expressway (new alignment for NY 100). This section would have ended by crossing NY 117 and Leonard Park into the Town of Bedford.
The next 7.0-mile section in the Town of Bedford, which was designated as a medium priority by the county, would have paralleled the existing NY 172 alignment. The NY 172 alignment was chosen for the route of the expressway because it lies in a natural valley, and the fairly steep slopes of either side of this alignment would impede highway construction. This section would have ended near Bedford Village at the intersection of the proposed Eastern Corridor Expressway (improved NY 22, near today's I-684).
Continuing east from the Bedford Village area, the NY 172 expressway would have crossed NY 137 (Stone Hill Road) in the vicinity of Pound Ridge. At this location, there would have been an interchange with the proposed Pound Ridge-Stamford Expressway (improved NY 137). The NY 172 expressway would have veered northeast toward the South Salem area, ending at the proposed Northern Westchester Expressway (improved NY 35) in the vicinity of Ridgefield Avenue. This final 5.0-mile section was designated as a low priority corridor by the county.
In 1966, Westchester County officials revived plans to construct an east-west expressway along the NY 172 corridor. The expressway would have connected I-684 with the proposed Hudson River Expressway (I-487) to the west, and with the NY 35 and NY 137 expressways to the east. The Ossining-South Salem Expressway proposal died quietly in the 1970's.