The unsigned Middlesex County segment begins at the intersection of Park Avenue (CR 531) and Maple Avenue (CR 602) in South Plainfield. The county route, not signed as CR 509 but rather CR 602, heads northeast on Maple Avenue through a residential neighborhood. After crossing Woodland Avenue, the road enters Edison and passes between two parts of the Plainfield Country Club. CR 602 breaks off the road at Inman Avenue leaving CR 509 on Old Raritan Road continuing northeast on a locally maintained road. At the Union County line, the CR 509 designation ends and Raritan Road continues into Scotch Plains.