Notwithstanding the provisions of § 46.2-1307, the governing body of Warren County may adopt ordinances designating the private roads within any residential development containing 50 or more lots as highways for law-enforcement purposes, and the governing body of Greene County, upon receipt of a petition therefore by a majority of property owners within a residential development containing 25 or more lots, may adopt ordinances designating the private roads within any such development as highways for law-enforcement purposes. Such ordinance may also provide for certification of road signs and speed limits by private licensed professional engineers using criteria developed by the Commissioner of Highways, and for law-enforcement purposes, such certification shall have the same effect as if certified by the Commissioner of Highways.