Why Fancy Destination Dispatch Elevators Often Perform Worse Than Simple Buttons
I explore the hidden complexity behind elevator algorithms, comparing basic SCAN and LOOK systems with advanced Otis RSR logic. While smarter re-optimization usually reduces wait times, I discovered that rigid Destination Dispatch kiosks often perform worse than traditional buttons because they lack flexibility. The data reveals that keeping things simple frequently beats over-engineering in everyday building traffic.
The kiosk enforces rigidity, you must get in the assigned elevator, and the loss in flexibility is not worth the extra information for the optimizer.
- peterldowns
Back in highschool, simulating different elevator algorithms was one of the projects I implemented during my CS class. It wasn't for the class — AP CS did not require anything like actual programming — but it was a fun project.
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!
- omoikane
> Destination Dispatch [...] are in general worse
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.
- woeh
I am currently staying in an AirBnB of a 60 floor tower, and the three elevator shafts definitely cannot handle this (I don't think the building was designed with renting to tourists in mind). The elevators are frequently completely saturated during weekends, meaning that the elevator stops at each floor, but noone can get in because the elevator is already full. This means the backlog of people wanting to use the elevators is increasing until after rush hour, and I have at one time experienced a waiting time of over half an hour.
What I take from this is, elevators need a proper check to see if they are full, and if so skip floors.
- dsego
In my residential building we have two elevators, a big one and a small one, the small one can fit 3-4 people if they squeeze together, it can't fit a bicycle. And of course, they don't work independently, so if the small one is already on your floor the big one won't stop there, so if I have my bicycle with me, I need to send the small one to a higher floor first, and since people are using the lifts constantly, sometimes I need to wait for a while and repeat the same process. There were times where the small car came several times before I managed to stop the large one.
- brandonpelfrey
For folks that have never seen elevator scheduling the game: https://play.elevatorsaga.com/
Enjoy this rabbit hole :D
- pavlov
One of the best-loved Maxis games of the 1990s was SimTower which was all about designing elevator flows for your skyscraper.
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.’
- hermanschaaf
Hah, I thought a lot about this problem while developing Sky Lobby, a mobile game (iOS / Android) about controlling and automating elevators.
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.
- sorz
I just come back from a furry con hosted in hotel in China, and learn how every furry con like this completely break hotel's elevators:
- 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.
- grishka
How often do people travel between non-ground floors though? In my own experience, whether it's a residential or an office building, 99% of elevator usage is a) someone going down to the ground floor, and b) someone going up from the ground floor. There are some exceptions like hotels that have amenities on different floors though.
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.
- olex
The biggest problem I always have with elevators is not the algorithm, it's the people seemingly being unable to grasp the concept of pressing either the up or the down request button, depending on where they want to go. Almost always I find someone will press both, because "then the elevator comes faster". Completely ignoring the fact that they end up going the wrong way first half the time, and adding an unnecessary halt for everyone already in there. How hard can it be to understand?..