Show HN: Elevators

A cool connection is that a spinning-disk hard drive (HDD) is actually kind of like one really long elevator, just wrapped around a spindle instead of perfectly vertical. The SCAN algorithm is actually a disk-scheduling algorithm!

https://en.wikipedia.org/wiki/Elevator_algorithm

I wonder if this is an artifact of how the author used random destinations. I worked in a building that used Destination Dispatch, and the common travel pattern seemed to be:

- Everyone who is not on the ground floor generally want to go to the ground floor.

- People who are on the ground floor generally travel in large groups to the same destination.

This happens because people who worked on the same floor often leave for lunch at the same time, and return at the same time to the same floor. Destination Dispatch helps in this case because it's batching large groups of people with the same destination.

In the game, some of the elevators are automated, and I wanted to choose an algorithm that best aligned with what players would expect an elevator to do. I ended up going with something close to LOOK, which (as the article states) is mostly what people expect. Except in case of ambiguity, I had it prioritise floors that have been waiting longer, to improve the p90, which is important in the game.

But now, add in these challenges:

- double-deck cabs (where attached cabs cover two floors at once)

- transfer floors between shafts

- express shafts

and the best (or at least, most expected) algorithm becomes much less obvious! I'll leave it as an exercise for the reader to figure out the best one.

Luckily I was in charge of a game, not a real elevator system, so I just had to find a heuristic that was good-enough, and could tweak the rules to let players manually override the elevator's plan if they don't like where it's going. This seems to have satisfied most players.

What I take from this is, elevators need a proper check to see if they are full, and if so skip floors.

An example iirc was waiting for luggage at the airport. It takes min of x minutes for the first bags to show. In one version, passengers went directly from the gate to the luggage carousel and waited x mins for the first bag → unhappy. Then the airport decided to add a long, round-about "walk" path from the gate to the luggage area that burned some of those minutes → same total "wait" but happier passengers.

I watch it like once a year because it always tickles some part of me. Like all the different modes you can get an elevator into. The most fancy one people might encounter is when moving into or out of a building. The front office can give you a key to give exclusive control over an elevator so your movers aren’t waiting around on elevators. Put it in that mode and it will stop responding to calls from other floors. Only the person with the key can control the elevator. You get on, select the floor, door closes elevator goes, and then just chills there with the door open waiting for you. Annoying for the rest if the building (the building is down an elevator when in that mode) but is amazing for the person using it! But there are way, way more depending on the installation and function.

Fun fact: most elevator shafts are sealed at the top as tight as possible to prevent them from becoming a giant chimney in a fire. It never even occurred to me until I was in a mechanical room wondering “where is the hatch to look down the shaft?” The answer is “there is none, and it’s a feature not a bug.” You want to block all airflow so fire doesn’t chase up the shaft into neighboring floors. Who knew!

1. https://en.wikipedia.org/wiki/The_Intuitionist

The first elevator "algorithm", if you can even call it that, that I've ever encountered and that seems intuitive to me, is what Soviet relay logic elevators did. If it's not in use and someone calls it, it goes there. If it's in use, it will collect the calls on its way down (and ignore them on the way up, these had no up/down call buttons). It's surprisingly effective for apartment buildings.

Or I guess you could do long-press to turn off or something if you're worried about confusing people, but I really don't think that should be necessary. We know how toggles work. The button lights up when it's on.

Of course, user error is also a factor, so this isn't accounting for people not understanding how to use it and making things worse that way.

Efficient elevator algorithms will often suggest moving an elevator into a preempted positioning strategy, based on time of day or peer elevator positions. The scheduler might move an elevator up to offset one coming down, for example. The demand-signal algorithms at least minimize movement in this way.

I think minimizing maintenance at the cost of increasing wait times is probably an important balance to get right. No doubt, the cost owners for maintenance are not going to perceive the wait time of passengers to be as important.

- rooms are all booked

- young furries usually share one room with 4 or even up to 6 friends

- more guest room-lobby-ballroom traffic

- far more inter-guest-floor traffic than their usual tourists

- one full-suit furry take 1.5-2 human's space

Their elevators are not designed for this scenario and got overloaded all the day. Queueing for tens of minutes is not uncommon.

There’s something so satisfying about watching a machine just dutifully work through queued tasks like this.

Even at that relatively reduced level of complexity, it's quite tricky to figure out how to prioritize who gets served when.

So, for instance, I didn't measure the elevator's efficiency in the wait time. I included (weighted) travel time. I assumed LOOK algorithm was used. And then when I tried to calculate various outcomes for different trips I suddenly realized that elevator's efficiency and fairness seem to go different ways.

Without trying to reproduce my evaluation system, here's my finding in fewer details: if you have two passengers, one going from the ground floor to the top floor and other joining for the shorter ride down in the middle, then the elevator that makes a "detour" is more efficient, but it hardly seems fair that the passenger traveling ground floor to the top should go even a single floor in the opposite direction.

Somehow, until that point, I lived with an illusion that the most efficient solution to a problem must be the most fair to every participant. Discovering this counterexample sent me on the "tour of discovery" of ethics and different philosophers who contributed to it... and while in the end it made me none the wiser, I'm happy to have discovered this field.

I swear no matter if it was busy or not, it seemed like everyone was assigned to the same one or two elevators. The wait was always so long too, I used the stairs most of the time if just going <6 flights.

Someone besides RCCL needs to study that algorithm.

It might be fun to revisit. The game was actually of Japanese origin.

The Digital Antiquarian just wrote about it the other week:

https://www.filfre.net/2026/07/the-life-and-times-of-maxis-p...

(Part of an ongoing multi-part Maxis article, SimTower is halfway down in this part 2.)

The origin story:

”As Yoot Saito would be the first to tell you, the rest of the game was really born out of his odd fascination with elevator systems.

> ’Late one night, I was waiting in my building’s lobby for an elevator. There are two cars in my building’s shaft. I pushed the button, but instead of the closest car coming to my floor, the farthest car arrived first. In my usual questioning way, I asked myself, “What happened here? How are these cars logically synchronized?” That was the beginning of SimTower.’

I can see how that could be challenging when people get off on intermediate floors, making a full car available again, changing status. And, if 3 people are waiting and there is room for 1, should it stop or not? Etc.

My favorite takeaway from this is how simplicity often beats complexity.

I wonder how do elevators find out if the car is full or not.

I once been to an office building where it obviously didn't work properly.

I needed to get down from a pretty high (15+) floor to the bottom during the evening rush.

The elevator got full immediately but still stopped almost on every floor on the way down.

Each time an awkward scene ensued: a huge crowd of people outside elevator just standing and looking at the crowd in the elevator. Everyone has to just wait pointlessly.

I visit a building sometimes with destination dispatch elevators, and with low exposure across multiple years I'm confused every time the elevator I need to take is open behind me.

I once used the paternoster lift in Germany, beats any elevator algorithm efficiency because it's a non-stop "conveyor" with no doors. I still get nightmares about that elevator.

I would have never thought there were so many challenges behind a car moving up & down.

This industry is so niche and peculiar.

Elevator trip scheduling is one of the many challenges. Really a great experience so far

> The state of the world 30sec after you called your elevator might be very different but the system is unable to adapt. Turns out the loss in flexibility is not worth the extra information for the optimizer.

Very interesting. I am on a 30 flr 8 car today so guess it made sense for Destination Dispatch. It does mean there is rarely a full elevator and usually 3 stops max allowing it to go fast to high floors.

Other things I thought could factor in elevators is shopping trolleys: both customers and trolley collectors. One level will have people coming in with full trolleys and almost always the other levels will not. I don't think elevators really care they just fill up and you need to call another one.

Also prams but they are 2-way.

Just ride to the floor I have access too, stay in while the doors close, and it would take me to a random floor. Push open door and voila I was on some random corporations private floor.

I figure something like a simple weighted ratio of car color = { 1.0: car fg color, max(1,speed^0.1)-1: shaft bg color } would do, so that 2x only shifts the color 7% towards the background color, but 500x shifts the color 86% towards the background color — and a stationary elevator can fade in from 86% bgcolor to 0% bgcolor over a few ticks as it sits there.

I recognize that for most of us, the flashing elevators blur into motion blur, but I can see individual frames at 120fps and while I am not seizure-sensitive to high-contrast flashing, I imagine those folks would appreciate you going the extra mile to reduce contrast at higher velocities, too.

As someone often using destination dispatch, I am inclined to think after the elevator arrives, you get to the destination more quickly than RSR or LOOK because it’s optimised everyone to the same floor (less stopping)

So it may be a case of more waiting in the lobby but less in a claustrophobic box, which I’ll happily take

I want something similar for advanced sensor stop lights...

I've long toyed with creating an elevator simulator game, but every time I build a prototype I realize it's not that fun to manually dispatch elevators, and playing with algos only goes so far! Elevator Saga is great for the programming side: https://play.elevatorsaga.com/

They have three elevators, but pressing one button only calls that specific elevator. So everyone presses all three buttons, meaning the three elevators basically turn into one.

I don‘t understand how this was ever signed off, but I‘ve seen it multiple times now...

[1]: https://clarkmoody.com/Moody_AgentBasedElevatorControl.pdf

https://jslandy.com/elevators/

I.e. in a bank of 2 elevators where one is resting on ground and the other is resting on a different floor, as soon as someone calls the ground elevator (which is already there and immediately opens) the other elevator (if idle) will move to the ground floor so that there's no delay the next time someone calls one from the ground floor.

What I find annoying though is you clunkily go click a floor then clunkily wait then clunkily walk over to the elevator assigned which may or may not be on the other end of the space. Also another interesting thing I've seen is that it seems in my case the distance from destination selection and elevator assigned seems to reduce at times when the office is emptier so that too seems to def be accounted into the algorithm.

https://simpy.readthedocs.io/en/latest/

Sounds like destination dispatch would work better if it didn't tell you which elevator to get into until the elevator arrived.

EDIT: already being discussed in the thread at https://news.ycombinator.com/item?id=49125423

On a related note, could someone please design continuous spiral escalators with horizontal sections at each floor?

0: https://en.wikipedia.org/wiki/Paternoster_lift#Safety

0: https://news.ycombinator.com/item?id=35152341

1: https://computer.rip/2023-03-13-the-door-close-button.html

I stayed in a hotel with these. It was horrible. Most people didn't figure it out, would get in the wrong elevator, and just get lost in the building.

Using such a primitive elevator gave me a newfound appreciation for the complexity of modern designs. The queueing algorithms are subtle but clever, and the difference in efficiency is instantly obvious once you've dealt with the alternative.

The simulator seems to have passengers spawning on a random floor and then transiting to a different random floor. But for where I work, the trips are pretty much always from the lobby to a floor, or from a floor to the lobby. Nobody goes inbetween floors except maybe the cleaners.

That will need planning and assignment I guess? But at what load figures does it become more efficient?

https://elevation.fandom.com/wiki/Sabbath_service_(SHO)

Where I live, on the taller buildings it is common to have banks of elevators which only service certain floors, eg floors 2:25 and 26-50 are two different banks of elevators.

Some modern elevators have two cars per shaft, imagine scheduling that.

https://github.com/tlaplus/Examples/blob/master/specificatio...

Another interesting, and underrated one: hotel keys.

Didn’t anticipate that my day would be bookended by relating to and empathizing with the plight of the elevators while I wait for one in a tall corporate building.

Pressed the kiosk some time ago but watching the sun begin to arc towards setting it does appear the world can change a lot in 30 seconds.

Patents, everybody...

For example, some elevators allow adjusting the door closing/opening speeds etc., I'd be curious what other things can be adjusted.

Now maybe I can beat that game!

1. https://www.dosgames.com/game/elevators-from-hell/

Them elevators are able to see into the future to be there even before we call them. Come on!

I love this by the way.

Missed opportunity to entitle it "Elevator Action"

Deviant Ollam on Elevator Hacking https://youtu.be/ZUvGfuLlZus

They have to be built a certain way, and need super fancy dispatch algorithms.

(In the example above we gave up and decided it was cardio time, five laps up and down 14 flights to escape with all our luggage.)

- john.fun vs neal.funnnnnnnnnnn

- who woooon? you decide!!!!!!

Level 2: Traffic lights, graphics (2D arrays)

Level 3: Flight control, simulations (3D arrays)

Level 4: Satellite tracking, virtual worlds (4D arrays)

...

So many problems can be cast to arrays to be solved with various linear algebra.

Considering LLMs are "Level 1000+" puzzles in this analogy, I wonder if every problem could be represented by an n-dimensional vector and solvable with algebra.

They at least make great interview questions - Tic-Tac-Toe is commonly given, but is obvious. The board already "looks" like arrays. The less obvious ones, like elevators (or load balancing), are always interesting.

claude could recreate it in unity in about 10 minutes. now ticked it off my "want to code" bucket list.

I was recently at a Radisson hotel in Germany where I was tried to summon an car, but none would appear for a long time. I believe that someone was keeping the doors open on their floor to wait for someone. The fact that all the other elevators were passing by in both directions was actually a bit infuriating as well.

At that moment I really wished for an algorithm that would recognize when an car spent a significant amount of time on one floor and then reassign the other floors to a new elevator.

- Two elevators ascending and descending in near lock step, in a two-elevator building, effectively reduced to one elevator.

- When going in a different direction from the requested one (e.g. called elevator to go up, but actually going down), the doors wastefully close, open and close again. Seen this in many elevators.

- At 3 a.m. you take the elevator to a parkade, like to get something from a car. When you return to the elevator a minute later, it is gone; it spontaneously moved away even though nobody is using it but you.

My takeaway is that the benefit of using a much more complex algorithm is marginal.

Really? Maybe I'm lucky I never had to deal a lot with elevators.

AAAAAAAAAAAH. WHY.

At a previous employer we had heteregenous elevators. In one bank, some went to the lower half of floors only while others went to all and the company installed a "smart" elevator system. I kind of became known because I was constantly yelling about this system in the feedback group because I hate "smart" elevators and and (IMHO) they just don't work. What actually works is express elevators and sky lobbies.

This article covers one of the deficiencies this system had: full elevators. For example, you'd want to go down and an elevator would skip your floor because another had already been assigned. That one would show up full and you couldn't go down. Down wasn't so bad because the stairs weren't a bad option but up was terrible. Going up a few floors was fine. Going up 20 was... a bigger issue.

Back in the day we had elevator operators and people in the lobby during the morning rush who would shepherd people into particular elevators. I actually think this system works way better than anything technology has come up with. Even if you nail the implementation (and I've never experienced elevators that have), people don't read and will just get into elevators anyway.

Anyway the article says that generally speaking on smaller banks simple up and down buttons work best. I absolutely agree.

There's a deeper issue here though and that is solutions looking for a problem. Nobody is making money from up and down buttons. They are fromn selling smart elevator solutions. And you see this everywhere in life. It basically devolves into rent-seeking behavior. Salespeople wine and dine a couple of people responsible for making decisions and then make bank on selling something nobody wants or needs as well as the constant maintenance and updates.

The core problem: elevators are frustrating for many reasons, and that frustration often manifests as hidden blame and hate towards other fellow passengers. Like, IDK, I'm sitting there with a stroller and a kid on floor -1, trying to get to the train platform at floor 0, but the elevator isn't coming because it's going back and forth between 0 and 2 and 3, as people who came by cars shuttle their luggage to and from the parking lot. As a city dweller and non-car person, I can feel my hatred towards cars and drivers bubbling up right there.

I imagine the same is felt by them if I manage to snatch the lift and go from 0 to -1, when they'd really love to already be on +2.

There's many other cases. Mall problems are different than train station problem, different than tall residential building problems, different than office building problems. Destination Dispatch is its own special thing, with its own special psychological twists. But two things I observed in general are:

- If there is a control element available, you will blame people for using it (even internally). Like, "I'd be there a minute ago if not for this damn XYZ who just had to press the button at exactly this time, and they weren't even going in this direction!"

- If there is an indicator element available, people will optimize around whatever it shows.

Like, someone gets at floor N, sees most of the buttons between there and their destination M lit up, immediately presses N+1 to get off there and call another elevator. But if the buttons are visible enough from outside the lift, they may just not get in in the first place, saving themselves and the others time and trouble, etc.

The more I think about it, the more I see that placement of buttons and indicators in the lift is in itself an UX engineering problem with unusually high impact of every decision on well-being of the users.

EDIT: Also ironically, and in line with my feeling that Star Trek got more things right than people give it credit for, a lot of elevator problems - both scheduling and social/psychological ones - would be solved if elevators just moved faster, like 2-4x faster than now, and also in some cases, also moved sideways. I.e. turbolifts.

(That, and if it didn't immediately led to the building owners putting less of them. So put another way: frustrating lifts is the sign of building owners/designers cheapening out.)

Beyond the content, the font, style, etc made it a pleasant experience for me.