Intensity of residential land use. The residential density allowed within the planned development shall be determined by the approved planned development site plan, except that, in any instance where a Planned development district occurs by rezoning of a prior residential district (R, RR-1 or RR-2), the density shall not exceed 200% of that otherwise permitted in the District Schedule of Area and Bulk Regulations[1] for that district. Clustering of residential units is encouraged with the requirement that not less than 40% of the total area of any tract developed as a PDD, exclusive of land area used primarily for vehicular modes of transportation and accessory uses, shall remain in perpetuity as common open space.