If the elevator stops on a floor, opens the doors to let the passengers out, and hasn't received a command to go on another floor, why does it close its doors afterwards?
If the elevator is not on my current floor then I can understand the reason you'd want to have the elevator doors remain closed - however if the elevator is already on my floor - it seems like closing the doors at that point is adding unnecessary time.
Is there a good reason for this?
A Google search doesn't really provide me any answers but I do have some ideas on why the doors would close:
1: Time Efficiency (As stated by Andrew, Ruudt, and Angelo) Closing the door reduces the time the elevator needs, to move to a different floor when someone presses the button.
2: Safety (1) When a door stays open and a user approaches the elevator, the door could close at the moment when the user enters the elevator, which could cause a problematic situation. Opening a door on request, lets the elevator know there is someone who wants to enter.
3: Safety (2) Generally elevators have double doors. One on the elevator itself and one on each floor, that opens whenever the elevator is at the right floor. To prevent people from falling down the shaft, both doors open at the exact same time via a mechanism. Keeping the doors open could possibly pose a hazard here of having the outer doors stay open while the elevator is not on the same floor.
4: Safety (3) Young kids, pets or wild animals are less likely to manage entering the elevator while it waits, since they won't be able to press the button.
5: Energy Efficiency Elevators tend to turn off the light when it is still waiting a new user to save energy. An open elevator with the lights off (to conserve energy) would give off the impression of a broken elevator, even if the lights were to turn on whenever a user steps in the elevator. Closing the doors and turning off the lights doesn't have this problem.
Disclaimer: I don't have research to back this up unfortunately. Just my two cents here.
Edit to conclude my list of reasons
It occurred to me that the main reason for closing the doors is an historical/cultural thing. Originally, doors were put in an elevator to prevent people from falling down the shaft. It's inventors weren't necessarily concerned about speed and efficiency; just safety. (Late 1800's)
Currently the main purpose of elevator doors still is safety. And although we have improved the technology over time, an open elevator poses possible threats because of faulty technology. Furthermore, it could also be perceived as more dangerous because we are not used to it.
Closing an elevator door is consistent with the behavior we believe is normal since that is the way they have been since the introduction of elevator doors. Keeping it open could induce a feeling of insecurity when entering the elevator.
Closing the door for the floor the elevator is on reduces the time, the elevator can serve another floor upon request. The time is saved due to omitting the door closing step.
Considering that elevators usually serve more than two floors this speeds up the more frequent use case.
There are several answers about reducing time for people calling the elevator from other floors, however while desirable, I believe that is a secondary benefit.
Safety trumps convenience.
Typically, automatic door closure is related to an emergency situation.
An example might be that a fire (possibly as yet undetected) has occurred on a floor , or hot or noxious gases are present for whatever reason at one level of the building.
Automatically closing the doors reduces the potential movement of smoke and/or gases into the lift, so that if the lift is called from another floor, the gases will not be brought from elsewhere only to flow out over unwitting waiters. This may include the possibility of the fire entering the lift itself.
This eventuality may still occur before fires or other issues have been detected and therefore prior to emergency procedures coming into effect.
EDIT Someone mentioned they've seen elevator doors remain open when on the ground floor.
This actually still fits the fire safety scenario, for example according to Schindler's Emergency Operation of Elevator Systems (yeah Schindler's Lifts!):
If a smoke sensor is tripped, either above or below the main lobby, all cars in that bank will recall, running nonstop to the main egress level and secure themselves with their doors open.
Thus doors remaining open at the main lobby reduces the time between an emergency being triggered and the final state being reached - (i.e. zero time)
It's simple math.
Let's say a building has 10 floors.
Leaving the doors open on the floor the elevator is on benefits 1 floor out of 10 as it reduces the time to enter the elevator.
But it lengthens the time to enter the elevator on 9 out of 10 floors as those floors now need to wait for the elevator doors to close before it can start moving.
As such, this would only benefit 10% of the people in the building at any given time but--even more important--inconvenient 90% of the people at any given time.
That's just bad UX.
From a user's perspective, the open door would also seem to be an invitation to get into the elevator. During the elapsed time of getting into the elevator, it could be called from another floor, causing the doors to close and at which point the boarding passenger thinks Wow! What a convenient and smart elevator!.
But, instead of going down to the lobby, suddenly they are on their way up to the penthouse because the calling floor button press had already occurred a few seconds earlier.
Elevators actually do have an option to stay on a floor and leave the doors open - that's what they do in service mode. But it also makes it ignore the call buttons outside the elevator.
It is a huge time saver for people moving in/out of an apartment building - the elevator will always stay with it's doors open on the current floor, and only close them when you select a floor on the panel inside the elevator. That gives the movers plenty of time to move things, and keeps the elevator ready for them for the next trip. In this case it's safe to assume they will be making a lot of trips from the ground floor to floor X, and back again.
But for normal operation, it's usually a better assumption that the next user will be on a different floor, and close the doors to be ready to move to them.
If an elevator is sitting idle on some floor with the doors open, then the time required to respond to a service request on that floor will be reduced, but the time required to respond to a service request elsewhere will be increased. In a building with multiple elevators, it may sometimes be possible to have one elevator sitting open at the main floor without impeding responsiveness elsewhere if other elevators are better positioned to handle requests from elsewhere.
Even in that situation, however, having the door stay open may not improve the user experience. If a door opens in response to a button push, a user may enter the elevator with confidence that the door isn't going to close instantly. By contrast, if a user arrives at an elevator and sees the door open, the user may have no idea how long the door will remain open. This may cause some anxiety which would have been avoided had the door been closed prior to user actions.
Further, time which elapses between a user's request and the start of the system's visible response is often more noticeable than time which elapses during the system's response. If the user pushes a button and the door starts to open 100ms later, users will perceive that the door opened instantly, even if three more seconds elapse before they can actually enter. Users will not perceive that the system cost them three seconds by not having opened the doors sooner. If having to close the doors before servicing a request from another floor adds three seconds to that other request, that three seconds is apt to be far more noticeable.
Finally, as noted elsewhere, if users do not expect elevator doors to stay open unless held, users are unlikely to act in ways which could pose a hazard if doors close unexpectedly. If doors are expected to close soon after they open, passengers who step out of an elevator will be inclined to immediately ensure that all of their luggage and associated straps are fully clear of the door. If doors often stay open, a passenger who isn't certain of where to go after arriving at a floor might step away for a moment to look at a map while a luggage strap is dangling across the threshold. In such a case, the doors might close on the strap, and the departing elevator might tear it off.
As with many other things, the answer probably lies in the possible failure modes.
Let us consider what can go wrong in various bad cases, when the doors are open, and when they are closed.
1) A loss of suspension. A cable snaps.
Doors open: you now have a gaping pit.
Doors closed: you now have closed doors that will not open again until the cage rises back to that level.
2) A fire alarm is hit.
Doors open: everyone piles into the elevator out of habit. This is about the most dangerous place to be in a fire, and, since elevators are disabled at this point, also the least effective escape route. You could simply close the doors in event of fire, to prevent this... but that risks trapping people inside the elevator.
Doors closed: nobody enters the elevator, because nobody can.
But there's also the matter of efficiency and responsiveness...
3) Someone attempts to enter the elevator just as someone on another floor summons it.
Doors open: the elevator responds immediately, closing the doors in the face of the passenger, as it has no idea they exist; or the elevator sounds a warning buzzer for a time, then closes the door, hence saving no time (and indeed, losing time if there was nobody there).
Doors closed: the elevator waits a while, as a button was pressed in order to open the doors on this floor.
4) Someone on another floor requests this elevator, with nobody on this floor.
Doors open: Time to respond = door-closing time, plus transit time, plus door-opening time.
Doors closed: Time to respond = transit time, plus door-opening time.
5) Someone on this floor requests this elevator.
Doors open: Time to respond = zero.
Doors closed: Time to respond = door-opening time.
Given 3, 4 and 5, unless 5 is in the significant majority of cases (such as on the ground floor) it is more efficient to have the doors closed. Having the doors closed on another floor also permits the idle elevator to move to the ground floor and open its doors there. The ground floor is not subject to the safety issues outlined above.
There are two programming modes where an elevator will stay open on a floor:
While there's merit to the safety and efficiency-related perspective (in fact, those reasons are likely why elevators do this), from a UX perspective there's a much more important reason for keeping elevator doors closed while waiting, even if all other considerations were removed.
It's about setting expectations to provide a good experience.
We all hate waiting and will always be disappointed when we learn that we have to wait. By forcing a user to push the call button before serving an open elevator cab, you defer the that initial disappointment (i.e. you don't see see the closed elevator from down the hallway and immediately experience the disappointment of your impending wait.)
Additionally, by forcing (and controlling) that initial interaction, you can continue to manage reasonable expectations. If a rider encounters an elevator ready for them to waltz right into, that's a statistical payoff. But people don't think in statistics: just as with game or task design, expectancy theory tells us that one payoff or "near hit" is enough to keep expectations high for a while. The downside here is that a user has a negative impression from all subsequent experiences that fall below the expectation.
Additionally, the further the reality of an outcome is from the expectation, the greater the effect will be, and humans hate losing more than they enjoy gaining. Having a controlled interaction point allows you to enforce a minimum wait that reduces the deviation in wait times and produces a smaller perceived loss for relatively longer waits, again managing expectations.
I suspect that it's mainly tradition, but all the other the reasons listed in the answers are incentive not to change it. That being said--here's one more reason. (I'd comment if I could.)
If you walk into an open elevator wanting to go down, someone might press the call button on a floor above you before you press the number you want to go to, forcing you to ride the elevator all the way up before going down. If there's only one elevator that's fine--but if there are two you really should be able to both get in different elevators.
Note that this isn't an issue if you're on the lowest or highest floor.
Another thing that hasn't been mentioned. A small pet such as a cat or mouse who manages to escape from an apartment could run towards an open elevator and fall down the small gap between the open elevator doors and the elevator. What a terrible way to die!
