Where an off-street parking area contains more than 15 parking spaces, it shall contain raised landscaped islands. Raised and curbed landscaped islands shall be located at the ends of each parking bay, and separating adjacent rows of parking spaces at least every second parking bay, as determined necessary by the Planning Board to properly guide vehicle movement, to provide adequate space for plant growth and vehicle overhang, to provide adequate space for pedestrian circulation where appropriate and to otherwise provide for improved traffic circulation, pedestrian safety and aesthetics. Such raised landscaped islands and the plantings within them shall be designed and arranged so as to provide vertical definition to major traffic circulation aisles, entrances and exits, to prevent indiscriminate diagonal movement of vehicles and to provide cooling shade and relief from the visual impact, monotony and heat of large expanses of paved areas. As an alternative to raised and curbed landscaped islands, the applicant may provide at or below grade landscaped areas that are incorporated into a stormwater control plan, where the design of such areas precludes vehicular entry by providing guardrails or partial curbs. Nonlandscaped islands will be permitted where appropriate to allow for snow storage, pedestrian circulation or other such purposes.