Elevators

I was just taking the elevator the other day and I noticed something:

Our block has three elevators, two of which are part of the same system and the third of which is separate. If you hit the elevator button, the elevator closest generally comes to you, unless it's going in the wrong direction. If you hit the button for the third elevator it doesn't affect the other two. The part that interested me was that two-elevator system. One elevator was at 10 (they generally stand at 10 and 0), the other was coming up. When I hit the button to go down on the 20th floor, elevator at 10 didn't move, presumably allowing the elevator coming up to serve me too. Unfortunately, when the elevator coming up hit the 10th floor (presumably the last floor for its passengers), it started going down! And the other one had already left down, so now I had no elevator coming up. I was in a hurry that day and so I chose the lift on the other side (the third one).

What I want to know is what algorithm these elevators choose to determine which one moves to serve whom. Does it use some simple go-when-called algorithm or does it take into consideration the fact that people on higher floors are more likely to be going all the way down. Given knowledge of when and in which order people are going to use the elevator, it is possible to solve the problem as to where the elevators should stand, but how would an adaptive model match up to this? I'm sure someone must have already thought of a similar problem, and I'd be very interested to read about it.

In fact I read about a similar problem once, though I can't recall what it was. It is nearly identical to this one. You have two operators, and three cities to service, but you do not know when/if each city will require an operator. The first two cities are close to each other (say 1 km) and the third (which is your headquarters and where each operator starts at) is 1000 km away from the other two. It may seem like the best option is to send one operator to one of the two cities and shuttle between the two when required but suppose the actual incidence of service requests are only in the two cities next to each other, you would have been better off choosing to send one operator to each of those. It's a bit of a muddled description but I think you can figure what I mean.