Why do some elevators have a single call button and others two?

Question

It seems like it would be optimal to have two call buttons (up/down) on an elevator instead of one because I could hop on an elevator headed in the same direction that I want to go versus having to wait for an idle elevator.

Is it because two buttons is more confusing since some users might think it is the direction they want the elevator to go instead of themselves? Or could it be because of the algorithm that most elevators use only makes use of idle elevators?

Answer 1

Two Call Buttons are used outside the Elevator to determine which Direction you'll be going and to avoid the elevator reach maximum capacity by putting request calls on HOLD by preventing to halt on floors where the person wants to travel an opposite direction.

Take this scenario, for example:

There are 10 floors in a Building.

The Elevator is on the Ground Floor.

5 people enter an elevator which can contain a total of 6.

All 5 people want to go up (obviously) to a floor above 7.

You are on the 5th floor. You want to go to the Ground Floor.

There's another person on the 6th floor wanting to travel to the 10th.

You press the Down button to indicate you'll be going down.

Person on the 6th floor presses the Up button to indicate he'll be travelling up.

Solution 1:

If we were to approach this problem with FCFS, you would get in the elevator, fill the maximum capacity, the elevator would stop on the 6th floor as well, since the other person called. He'd notice that it's full. Elevator will close and go up. You'll then wait until all the other 5 people go to the >7 floors. Elevator would come down back to the 6th, take the other person, travel all the way to the Ground floor and then take the other person to the 10th.

Not optimal.

Solution 2:

We approach this problem with the direction the person who wants to get in the elevator wants to travel to. Since the Elevator's direction is now from Ground -> 10th floor, it will place your request on Hold, since you want to go the opposite direction.

It will go to the 6th floor and halt for the person who wants to travel in the same direction. Elevator is now full. Everyone wants to get down at a floor >7. Elevator goes up, till the 10th. Now it's empty. It comes down, and now takes all the held requests back in consideration.

You want to now travel in the same direction. It stops on your floor and let's you step in and takes you to the Ground floor.

These are used in cases where there is a lot of weight to be carried into the Elevator and might reach maximum capacity faster. For eg. Hospitals.

Optimal.

Single Call Buttons do not pay heed to which direction you're going and approach the problem by FCFS.

This is normally used in Residential Buildings since the capacity isn't normally reached.

Answer 2

The purpose of the two call button (up and down) was based on the elevator algorithm, used in aiding the efficiency of transportation (https://en.wikipedia.org/wiki/Elevator):

• Continue traveling in the same direction while there are remaining requests in that same direction.
• If there are no further requests in that direction, then stop and become idle, or change direction if there are requests in the opposite direction.

On base and top floor, a single button was used.

With elevators systems that have only one external control, are usually where the algorithm is not needed (such as only having a few floors).

Answer 3

After reading the question, I have paid some attention during the weekend and found various instances of elevators that do not have separate call buttons for up and down in Germany. Given that Germany is a rather industrialized place, and the buildings in question were built at earliest in the second half of the 1970s, and rather later than that, I quite doubt (see also below) "limited technology" is a reason in this case, as hinted at in some comments.

However, I have noticed a bit of a pattern: The elevators with only one button that I did find were parts of train stations. (see the EDIT below for an update on this)

Here is an example of one (showing level -1 on a 4-floor scale from -2 to 1), whose year of construction is indicated as 2005 in the elevator:

Contents of the control panel, from top to bottom:

• direction up indicator
• direction down indicator
• malfunction/out of service indicator
• call button

This is why I suggest the following possible reasons:

• The cost factor may play a role. While the initial installation of a two-button system is well feasible, I am rather thinking of the fact that these elevators are in public places, some even in the outside. Hence, all external interface elements are subject to weather conditions (including 30°C of heat and -10°C with snow throughout the year), and possibly, vandalism. Therefore, you would want to keep the number of individually breakable parts that may need to be replaced as small as possible, and having one button rather than two may make sense in this respect.
• The overall number of floors is limited. In all, the elevators in question would serve only 3 or 4 floors. As such, even if you want to go from a middle floor to the lowermost or uppermost floor, and the elevator first takes you towards the opposite direction, the duration of the detour is limited.
  • Speaking of which, if you really want to avoid a detour, while you cannot call the elevator to travel towards a specific direction, there is a direction indicator (the white arrows in the photo) that shows where the elevator is currently going. This is one more reason why I severely doubt the "limited technology" argument here, given that if the elevator "knows" where it is going, making the elevator stop only if it is going towards the direction it was called for is trivial in terms of system logic.
• There is no strong sense of floors in the respective buildings. In theory, each floor has a number. However, travellers, especially those who do not know the place, do not want to go to "floor 2", or "floor 4", or "floor -3". They want to go to "long-distance trains", or to "regional trains", or to "subways", or to "exit on West Plaza", or to "exit towards city center". Neither do they know nor care especially how these respective platforms and exits are ordered vertically. Combined with the aforementioned points, my suspicion for the described scenario is that it would be unnecessary cognitive overhead to force travellers to think in vertical directions.

EDIT: I have meanwhile consciously encountered numerous other elevators with only one call button. Elevators, that were not located in train stations, and that were located in buildings with numbered floors.

A counterexample to my above points?

Actually, no, not at all - because the buttons that I did see were located in parking garages. Despite prominently featuring numbered floors, there is no strong sense of floors in this kind of buildings, either:

• There is little to no pedestrian traffic between individual floors in the building. People do not normally want to travel from level 3 to level 5, or from level 4 to level 1 in a parking garage. They have parked on one of the levels and want to leave. At this point, the only objective they are interested in is finding the exit, and they do not really need to know whether the exit is located above or below their current level.
• Likewise, when entering the parking garage, pedestrians will know the level number they want to go to. However, once again, they do not necessarily know which level they are currently on/which level they entered from.