I've asked myself the same question for a long time. Have you tried maybe camping out in an elevator for a day and see what happens?
One observation that I've made is that the elevator in my department building doesn't always idle on the first floor - I think that it generally sits on the second or third (out of a five-story building + basement). Just on a first thought, perhaps one way to make elevator operation efficient is to set the idle point to the "centre of mass" of all of the people in the building. It would seem to me that the average transit time for an elevator call would be much less than if the idle point were at the top floor or something extreme. (I haven't run any numbers or anything like that.) One concern I have is that optimizing this factor alone probably isn't enough, and could possibly make things worse, depending on elevator usage patterns.
Have you had any ideas on this matter?