Sensor Lighting in Stairwells

In order to reduce energy costs, we are looking at installing lighting fixtures that have motion/proximity sensors for when there is no activity (they will drop to a night light mode until someone enters the stairwell again).  Are these allowed by code?

Yes, they are permitted. Shown below is the applicable reference from the 2000 edition of NFPA 101^®, Life Safety Code^®:

19.2.8 Illumination of Means of Egress.

Means of egress shall be illuminated in accordance with Section 7.8.

7.8.1.2

Illumination of means of egress shall be continuous during the time that the conditions of occupancy require that the means of egress be available for use. Artificial lighting shall be employed at such locations and for such periods of time as required to maintain the illumination to the minimum criteria values herein specified.

Exception:  Automatic, motion sensor-type lighting switches shall be permitted within the means of egress, provided that the switch controllers are equipped for fail-safe operation, the illumination timers are set for a minimum 15-minute duration, and the motion sensor is activated by any occupant movement in the area served by the lighting units. of egress shall be illuminated in accordance with Section 7.8.