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We've all seen it in the movies where a kid will push all the buttons in an elevator. We've all likely done it - accidentally hit the wrong floor.
There's no unselect after you select a floor in an elevator.
Why is that? It seems that it would be a useful feature but is there some sort of logical or UX reason why you would not implement an unselect feature?
I agree with toscho, but I'd like to add another argument: It can cause problems with the next person entering the elevator. People often just hit the button for the floor they want to go to, paying no heed if that button has been selected already (if such feedback is clearly visible at all). If you'd allow an unselect, neither of the people wanting to go to that floor will arrive there. That is, unless you add a whole new set of buttons for the unselect, but doubling the amount of buttons won't improve usability...
I beg to differ - I've been in plenty of lifts where you can deselect a floor. It operates by holding onto the button for a few seconds until the light goes out (or sometimes by double-clicking the selected floor button to unselect it). It's possibly done in this way to preclude the problem @André was referring to - forcing people to make a deliberate choice to deselect rather than doing it accidentally.
Then we would see kids in the movies unselecting all buttons. Since nobody is hurt if the elevator stops too often, it may be better to keep the interface as simple as possible. Hence, no unselect button.
I have seen elevators with an unselect option where pressing the floor button after selected deselects it. However I believe the primary reason behind preventing people from cancelling a selection was to account for the case of an user entering the elevator and proceeding to cancel all other selected floors in an hurry to get to his destination (or just because he felt like being a jerk).
The lack of a cancel button would also force people to pay attention to their choice as there was no apparent revert option hopefully preventing a reduction in a number of random selects
We have an elevator at work with two sets of numbers on both sides of the door. Also when you click a selected floor it deselects it. Many times it happens that I press my floor number and someone else comes in immediately after me and presses the same floor number on the other set of numbers, thus deselecting it.
There is actually an elevator museum in New York where you can learn all sorts of interesting facts around the evolution of the elevator.
I learned that there is a scheduling algorithm that tries to keep all the cars in the right place at the right time based on previous traffic patterns. Anytime a user interacts with an elevator they are messing with this flow. Because of this many elevators are wired to ignore you.
Having unselect option increases complexity and requires more scenarios to be accounted for, For instance, if the lone selected floor button gets unselected unintentionally, will the elevator halt mid-way? There is no critical use-case for unselect. Like @toscho pointed out "Since nobody is hurt if the elevator stops too often", there would be a slight delay but nothing critical.
So its best to follow the KISS principle!
It's quite commonplace in Japan for lift buttons to have an 'undo' function - press to select, press to un-select - but I've never encountered it in the UK ever. Maybe it's because I'm not holding the button down for long enough over here, but I'm certainly going to try it from now on.
Im some elevators there's a "Stop" button.
This button is usually marked with a red colour and placed nearby the alarm/emergency button.
If you press the button you can freeze the elevator and eventually make another choice.
It seems to be a discouraged action (location, warning colour) but it works..
Years ago, some elevators had buttons that stayed pushed in until you arrived at your floor and then popped out. Most of these could be deselected by simply pulling the button back out.
I think, historically, the usability was restricted by the scheduling algorithm. If you cancel a floor selection, the scheduling algorithm has to re-calculate its schedule. Depending on the simplicity of the algorithm, this may not be possible/worthwhile. For an overview of factors that are taken into account in elevator scheduling algorithms, have a look at this Quora answer.
I believe the reason why modern elevators are supporting cancelling floors is simply because they have better scheduling algorithms that allow for this added complexity.
"Nobody gets hurt by the elevator stopping too often".
Hmmm, obviously you haven't seen the "road rage" that can ensure from poor elevator service. Sure the people in the elevator aren't harmed, but I won't vouch for the people standing next to someone waiting for the elevator.
