In 2002, we briefly reviewed the benefits of bi-level stairwell lighting, which uses built-in ultrasonic occupancy sensors to dim light levels and cut wattage while stairwells are unoccupied. Studies at various facilities found occupancy as low as 2% with variations depending on type of building (e.g., apartment house, dormitory, commercial offices).
Such products have now been in service long enough to provide a high level of confidence, and independent studies are available supporting their use to cut energy and maintenance costs. Some states offer incentives for installation of such energy-saving devices.