Tank vehicles wherever parked shall be positioned for immediate drive away or towing and a clear space of not less than 15 feet shall be maintained between any tank vehicle and any similar or other vehicle. No more than four refueler tank vehicles shall be parked unattended at any one location. Where five or more vehicles are parked, a constant attendant, qualified to drive the vehicles and competent in the operation of the fire protection equipment, shall be stationed in the area observing the operation and 150-pound dry chemical wheel type fire extinguishers shall be positioned so one or more units will be within 100 feet travel distance from any parked vehicle in the designated area. Satellite parking areas shall be posted from all directions with a sign legible from a 100 feet distance reading, “Danger—Aircraft Fuel—No Smoking—No Open Fire or Lights.”