site: reviews — Kawran Bazar, Elevators, Cycloidal Gearbox (graduate from finds)

src/content/reviews/elevators/index.mdx

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+---
+name: "Elevators"
+subtitle: "An interactive case that the fancy elevator kiosk is making you wait longer"
+date: 2026-08-02
+category: learning
+topics: [web-tools, learning-and-reference]
+tags: [interactive, algorithms, explainer, free, simulation]
+link: "https://john.fun/elevators"
+linkText: "john.fun"
+description: "A browser explainer that simulates elevator dispatch — SCAN, LOOK, Otis RSR and destination dispatch — and shows the destination-dispatch kiosk usually losing to plain up and down buttons."
+source: "Hacker News — Show HN"
+fromFind: "elevators"
+---
+
+import { Image } from 'astro:assets';
+import hero from './hero.png';
+
+<Image src={hero} alt="A schematic of six elevator shafts drawn on pale graph paper, floors numbered one to four, with dark blue cars sitting at different levels and small orange dots marking waiting calls inside them." />
+
+> Every algorithm in this piece runs live on the page, which is how it gets away with an argument you would not otherwise believe: the destination-dispatch kiosk in the smart building is probably making your wait worse.
+
+The find sold this short. SCAN and LOOK — go to the top and reverse, versus go only as high as anyone asked — are the first two paragraphs. What follows is applied statistics with the simulator wired into every claim.
+
+It starts by refusing the obvious metric. Nobody remembers their average wait; they remember the morning the lift took forever. So the piece works in **p50 and p90**, and shows both moving with traffic shape — the morning rush upward from the lobby, the evening flip, the lunch mix, interfloor. Morning is the worst, and you can watch it be the worst.
+
+Then it builds up Otis's **RSR** dispatcher, which scores every car for every call: time to reach you, plus a penalty for how loaded it already is, plus an anti-bunching penalty if another car is already heading to your floor in the same direction, minus bonuses for matching your direction, sitting idle within two floors, and running light. Crucially, RSR re-optimises every five seconds — the car coming for you can be swapped for a different car mid-journey.
+
+Two results earn the whole thing. First, **RSR does not always win**: at high traffic, when every car is full and stopping everywhere anyway, plain LOOK catches up and passes it, and LOOK also does better in small buildings with few cars per bank. Second, **destination dispatch — the kiosk that assigns you a car before you board — is generally worse**, and the reason is exactly the five-second re-optimisation it gives up. Telling the system where everyone is going sounds like more information; committing each passenger to one car is less flexibility, and the flexibility is worth more. Only very tall buildings with eight-plus cars per bank flip it back.
+
+**One date to treat carefully.** The piece opens by dating SCAN to a 1961 patent; Wikipedia's treatment of the elevator algorithm names no patent and puts the earliest published description in Knuth. The 1961 thread leads somewhere better anyway: to Leo Port, an Australian engineer credited with conceiving destination dispatch, who died in 1978 — decades before anyone built one, and remembered for the idea this article spends its last section dismantling.
+
+It ends with the full simulator — floors, cars, flow rate, all three algorithms, histogram on — and the only correct closing line: the elevator did hear you, it just has a lot to think about.