I often wonder why most elevator buttons aren't toggles or de-selectable buttons, so they can be de- or re-selected according to our up-to-the-moment thinking. Perhaps you press the wrong floor button. Let's include the call buttons outside the elevators, so an impatient citizen can cancel the request when he decides to use the stairs and you don't arrive at a floor with no one there.
This would not only save power, but also our precious time and nerves.