No tract within the Arterial Corridor (Overlay) District shall be provided with more than one point of direct access to the arterial. Unless, and only if, one accessway is impracticable in the judgment of the governing body, two accessways [to an arterial, such as PA 309] may be permitted, provided they are separated from each other and from any accessway on any abutting property and from any intersecting street by a minimum of 500 feet, as measured along the frontage of the arterial. Where such spacing cannot be achieved, no more than one accessway shall be permitted, except that a second limited purpose (i.e., right-in and right-out only) accessway may be permitted by the Board of Supervisors where the applicant demonstrates the need for the second access and as long as there is a minimum spacing of no less than 350 feet.