Outside the elevator: button array
There is an argument to be made for using traditional physical elevator buttons rather than a dynamic touch screen interface. In this design, the array of plastic buttons found inside traditional elevators is moved outside, to the elevator lobby. Users press the button for their floor, and the button's border glows with the distinct color of the elevator that is being summoned for them. The letter designation of the summoned elevator appears in the center of the button. Each of the panels in the elevator lobby reflect the same information
Learnability: People are very familiar with arrays of elevator buttons like these, and the process of selecting a floor is very simple. Where users might get confused is in boarding the correct elevator. The color should be sufficient to indicate the correct elevator to board. For elevator lobbies with a very large number of elevators, and for the colorblind, the letter which appears in the center of the button should make finding the correct elevator possible. Matching people to their correct elevators is also in part the job of the displays above the elevators, which are designed in a separate document.
Efficiency: The interface is simple and therefore can be operated very quickly. It also doesn't display any information besides the user's target elevator which prevents users from crowding around the interface, which would slow down arriving users.
Safety: If the user presses the wrong button, there isn't a way to undo their selection, which does slow the system down. However, in a building with a lot of elevators this would not have a very significant affect on the user's wait time. If the cues directing each user to their elevator aren't clear enough, users might end up taking the wrong elevator.
Inside the elevator: minimal display
In this design, the display inside the elevator shows only a list of the floors the elevator will stop at, and the estimated time until the next stop. The only part of the interface the user can interact with is the emergency exit button, which brings the user to the closest floor and opens the doors, while sounding an alarm (to prevent abuse).
Learnability: With only one button which will only rarely be used, this interface rarely requires action from the user. Assuming the user operated the outside interface correctly, once they step into the elevator they are only required to wait. The information displayed on the screen is very straight-forward, and shouldn't require any kind of documentation or instructions.
Efficiency: No actions are required by the user. The floor numbers on the display are displayed in either ascending or descending order, so it is easy for the user to locate their floor in the list.
Safety: Because the emergency exit button sounds an alarm, in most of the cases where a user gets into the wrong elevator they will just have to wait until the elevator reaches its destination, where they can call a second elevator. This is an unfortunate side-effect of our system, but we believe that any other system would lead to abuse of our system.
Next to the elevator: above
This elevator outside display is located above each elevator, to increase visibility and to make it clear that it is distinct from the floor-selecting interface. It displays a list of the floors the elevator is preparing to visit, in ascending order. It also displays the elevator's ETA.
Learnability: The meaning of the numbers of the elevator isn't particularly intuitive to a user unfamiliar with the system. But this is not a very good place for any sort of documentation; the other pieces of the system should have educated the user by the time the user needs to look at the display above the elevator.
Efficiency: Because the floor numbers are ordered, searching for a floor number is quick.
Safety: (Not applicable to this interface)