In the case of a major street, the Planning Board, in connection with site plan approval, may require the installation of a marginal road where such road is deemed necessary to the maintenance or improvement of the flow of through traffic on the major street. Such marginal road shall have a right-of-way of 40 feet, and shall include therein a thirty-foot paved roadway. In cases where the Board shall require such marginal road, the building setback shall be measured from the right-of-way line of the major street. For the purposes of this ordinance, a major street shall be any state or county highway, or other street so designated on the Town plan.