The middle section of NY 100 uses the southernmost segment of the Briarcliff-Peekskill Parkway, which is a continuation of the Saw Mill River Road, between Hawthorne and Briarcliff Manor, sharing it with NY 9A. The Saw Mill River Road splits from the parkway along with NY-100, and NY-100 uses the Saw Mill River Road until it splits from NY-100 in the Town of Yorktown to follow NY-118. The Saw Mill River Road is an early colonial road connecting many different hamlets and villages in Westchester County. It follows along the path of various rivers and brooks, the most notable of which being the road's namesake, the Saw Mill River, as it winds its way to the north of the county. The road is now used as parts of several state routes, including NY 9A, NY 100, and NY 118. The segment used by NY-100 mostly follows the Pocantico River.