Open space land shall be set aside and developed for the common use and enjoyment of all residents in a planned unit residential development area. The amount, location and proposed use of all open space land within the site shall be clearly shown. Of the gross area of open space land, a minimum of 40% shall be suitable for active recreational purposes, and access to these open space lands shall be convenient to all residents. For purposes of calculation, streets, sidewalks or land lying within 10 feet of any townhouse or garden apartment shall not be considered as open space land.