The lights in our stairwells are sound sensitive, meaning they come on whenever you make a loud noise. It is the same idea as the 'Clapper" that was popular in the Eighties, It is not just the stairwell in our building, but everyone around the entire country. As you sit in your apartment you can hear different people ascending and descending the stairs using their own unique ways to get the lights to come on. Some people stomp (I find this hard on the knees), others can make a high pitch whistling sound, I personally find coughing/clearing my throat the most effective noise to make to turn the lights on.
The other night as my friend and I were walking home late at night we noticed that many of the street lamps on our block were out. Without even realizing it I started coughing loudly hoping more light would come on. Apparently I an not the only one that has this reaction to darkness. You could hear other pedestrians clapping, whistling, stomping and the such, in hopes of lighting their way home.