What escalators do

According to Tom Vanderbilt in his walking series in Slate:

Contrary to what you might expect, escalators do not actually improve efficiency. “If you count the per square foot width on an escalator, plus the acceptance rate”—i.e., how willing people are to stand near each other—“it’s about the same as stairs.” And when people walk on an escalator, the capacity is, a bit counterintuitively, reduced, because of the subsequent shuffling—and accidents. (Escalator walking was actually banned in Japan.) What escalators do, simply, is reduce the amount of energy people have to exert (and during rush hours, an escalator is often the only way people are assured of being able to go against the cresting tide of directional flow).

More here, including great slide shows.