### Why do idle elevators close the doors?

• 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?

My old dentists office had physical doors which could only be opened when the elevator was on that floor (some kind of locking mechanism).

Not all elevators behave like that. I've seen elevators that keep the door open when on the ground floor, where many people want to enter.

There are elevators which don't have doors at all... http://en.wikipedia.org/wiki/Paternoster So the question could be rephrased about the need for having user operated doors in the first place rather than an automatic guard or fitting the inside of the elevator shelf at the door side with a seemless, low friction material.

Actually, the usual duty of an elevator is to carry people one way and return empty. In the morning most people go up, in the evening most go down. To me, the normal thing to do would be to close the doors and go back to where they were last called.

"open doors" = "broken/currently serviced elevator"

@FlorianF That would waste energy. If the lift started on the ground floor(floor 0) and went up to floor 4, then went back to the ground floor before being called up to floor 2 it would pass through a total of 10 floors (0>4 = 4 changes, 4>0 = 4 changes, 0>2 = 2 changes). If the lift stayed where it was when people got off, the journey would then be ground floor to floor 4, then to floor 2, which is only 6 floor changes. It might make sense in a smaller building, but it's still got the potential to waste energy.

@Pharap: more important than optimizing energy for elevator algorithm is to optimize for travel latency/delay (between request to arrival) and starvations/fairness (can't have people in the top-most floor always being the slowest to serve). In busy offices where the direction of travel is highly asymmetric during different times, elevators may be programmed to have most of the cars wait in the ground floor in the morning when most people are going up and to spread across higher floors in the evening where most people are going down.

@LieRyan It's wasted energy and time. If someone calls the lift to floor 6 from floor 0, then the next call is from floor 8, the time spent travelling 8 floors is less than had it just stayed at floor 6. Though technically it would probably make more sense that the lift should always return to the centre of the building so it takes equal time to reach the top floor or the ground floor.

@Pharap: in most places, the entrees and exitees are not uniformly distributed but rather follow a predictable pattern. Suppose that you know that at certain hour, 95% of users will be coming from GF going to a uniformly distributed destination and the 5% are uniformly distributed; if the lift waits on its last destination floor, then 95% of people would have to wait for the lift to move N levels (N/2 to go to GF and N/2 to go to their destination). OTOH, if the lift has a home floor, 95% of people only have to wait for an average of N/2 floors. Also lifts needs to accelerate to speed.

@Pharap: another case, suppose a really famous restaurant is in the penthouse of an otherwise sleepy office building. Most people eating there are outsiders coming from GF, rather than the office workers. During lunch time, it would make sense to always have one lift stationed on the penthouse and one on GF, as probabilistically those are the highest traffic area. Also, serving the penthouse the fastest will probably leave the lift with room to spare to pick up passengers from other floor, while additional delay on those floors can mean that the lifts will always be full on GF and penthouse.

@gerrit is right, some elevators will leave the door open on the ground floor so that people can just walk in, but on other floors it'd be less common for people to walk up to the elevator on the non lobby floors.

It's more complicated than that, says the manual for Otis elevators

@Florian - In home buildings, it is the contrary. In the morning most people go down. And in the evening most people go up.

Indeed, this is a very good question. Interestingly, the lifts with a manual outer door leave their inner door open when idle. The manual outer door typically has a window. The automatic lifts in the Paris métro and RER keep their doors open when waiting. These both cases offer the advantage of seeing whether the lift is there.

The fact that the doors always close when the elevator is idle makes it hard to air it out when a smoker has just taken it directly after cigarette break outside. So I end up punching all the buttons at once, hoping the repeated open/close will perform the same function.

7 years ago

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.

point 5 is quite interesting! a pet could accidentally enter an otherwise empty elevator and end up lost at another floor.

I'm not entirely certain I buy the 4th point. How does the elevator know no one is in the elevator?

@nhgrif - I once entered an elevator deep in my thoughts and forgot to press button to go up. In 10 or 20 seconds the lights and fan went off. Elevator does not need to "know". An inactivity timer works just fine.

I don't buy point 2 (Safety 1). If elevator doors close within a limited timespan after opening, the probability that the doors close right when someone tries to walk through is even higher than that of entering an open elevator at the same moment as the elevator happens to be called on another floor. Just think of elderly or otherwise slow people, multi-elevator systems where you have to bridge a few meters from the central calling button to the farthest elevator, or large groups of people who do not make it in time through the elevator doors.

@nhgrif - I wonder if some lifts might know that they are occupied by the weight increase (all lifts will check they are not overloaded before moving, and alert/alarm if they are overloaded anyway, so weight must be measured!)

Most elevators have sensors that detect when a hand or other object goes between the doors. Older elevators had a bar that acted as a switch so the door would have to close on you to activate the switch. Both systems sometimes work poorly. I've had an elevator try to close on me 20+ stories up... not fun.

@ O.R. Mapper: Point 2 is indeed minor but it is still a valid reason for shutting the door. As a user you know that when the door opens on request, or you see someone entering the elevator, you have x seconds to enter before the doors try to close. If the doors are always open you never really know when they will close. The behavior becomes unpredictable. If the sensors work poorly/slow this could mean that you get hit by the doors before they open up again.

I like point 4, imagine calling the elevator, it reaches your floor and a velociraptor charges out! You'd be wishing they didn't keep the doors open on the Jurassic floor for sure.

What about the case of a creepiness factor. I mean have you had to ride an elevator without doors? (Service Elevator or the like) when it reached each individual floor I got very anxious about the possibility of something going wrong. Granted, I'm afraid of heights, so seeing the actual walls in between each floor had an eeriness to it.

Even if it could be easily implemented (since elevators have scales), I doubt point 5 *-the edited one-* is right... I believe in most modern elevators the lights are always on. This can be easily proven since many elevators use fluorescent lights which may take a while to turn on, and you don't see them turning on when the gates open on pressing the button (if the elevator is already on your floor). A waste of energy? yes... but also reality.

@QuestionMarcs I guess that's something people would get used to. In my country, it's not that uncommon to see elevators with no or malfunctioning inner doors. Also almost all elevators have windows on inner doors, so you can see the wall in any case and I've never heard anyone of commenting that it's distracting to take a ride in such elevator.

I can tell you that elevators don't turn off the lights when idle (lights are required to be on), however do actually turn off the fans inside.

Safety 2 is moot. An elevator cab will never take off unless both doors are closed, and having the inner door closing while the outer door stays open is mechanically impossible (there is no motor on the outer door, when the cab is on the floor the inner door's mechanism also moves the outer door).

@AndréDaniel However, what if the sensor fails? If the door is closed by default, both these things would have to happen to have the door open after the elevator has left: (1) the functioning motor would have to suddenly open the door, (2) the door sensor would have to fail. With the door open by default, only one of these need to happen.

@Derek朕會功夫 Depends on the country I think.

@yo' there are two separate sensors for the outer and inner door, if one fails its signal won't match the other's signal and the elevator's controller wouldn't allow the cab to move. Also I believe the door locking mechanism itself can give feedback and the controller can know whether the door locked successfully.

This is a really good answer, but @Kent's answer below makes a great point that isn't covered here: What happens if you've just walked in to an already-open elevator, haven't pressed a button yet, and it gets called elsewhere? It's a classic race condition, albeit on a much slower scale than software usually has to handle.

About the point 2 : A lift that closes its doors can also have someone walking in at the time of closing. And the person often walks in without pushing the button, so the lift does not know that someone wants to come in. A person coming in while the lift is closing is a common situation that lifts usually are able to handle.

The point 4 can be a good reason for cats and children. On the other hand, requiring the person to push the button can be an annoyance. When the person is old, or has many packs, or is moving furniture...

Regarding the point 4 : Closing the doors can be *a false good idea*. If the doors stay open, the cat or child can come in, but then s/he can go out, especially when the light goes off. Now, if the doors close, the cat or child has a smaller risk of going in, but s/he can go in anyway, and then s/he has a big risk of being trapped inside and ending up later at another floor.

@GWv - With a lift keeping its doors open, if you were the anxious type, you could always begin with pushing the button. Thus, you would be sure that the lift would not close on you.

The point 5 is not valid. If a user thinks that an unlit lift is broken, it is a user problem. Some people think that because lifts do close their doors. If lifts work differently, people think differently. For instance, lifts with a manual outer door typically have a window on the outer door, leave their inner door open and turn their light off after a while. People take them anyway. People coming in an unlit flat don't think the flat is broken.