2019 explainer on traffic signal engineering resurfaces on Hacker News
A Practical Engineering blog post from 2019 explaining how traffic signals work climbed the Hacker News front page and spread to Reddit and Mastodon.
What to know
- A 2019 explainer on traffic signal engineering resurfaced and reached the Hacker News front page on 2026-09-21.
- The article discusses why intersections, not road capacity, limit urban traffic throughput and how signals manage right-of-way.
- The resurfacing generated cross-posts on an HN RSS aggregator, Mastodon, and Reddit, but no distinct new reporting or reactions beyond the original technical content are present in the evidence.
at1as Hacker News submittertw1st3d_m3nt4t Reddit submitterPractical Engineering Original publisher of the 2019 explainer
How it unfolded 3 developments, newest first · click a bar or a number to jump articlesposts
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Reddit user shares the same article separately
A separate Reddit submission of the same Practical Engineering article appeared under the title "How Do Traffic Signals Work?", posted by tw1st3d_m3nt4t.
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Historically, the amount of data available to traffic lights controllers was rather rudimentary, basically a simple presence/occupancy detector covering the area immediately upstream of the stop line and/or some point somewhat (a few seconds of driving time) further upstream of the stop line and/or the whole area in-between. The detector could be…
2 more of the top 3 · 15 posts in this stretch
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I think you mean "robots" ;).Seriously though, speaking as a South African living basically next to Sandton, there are plenty of traffic lights near to me which cause complete chaos when they are out. They will cause significant delay for the entire working day on certain routes, e.g. anywhere on Winnie Mandela (formerly William Nicol) Drive.There…
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Many progressive cities intentionally time their lights for this for two reasons. One is to intentionally limit capacity. Once a road gets past a certain density, you end up with problems that make things astronomically worse (for example: people who "block the box" and cause everything to grind to a halt.)The second reason: trying to incentivize…
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Mastodon bot account cross-posts the HN submission
An automated Mastodon account (h4ckernews@mastodon.social) reposted the article link along with the Hacker News comments URL and topical hashtags.
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> Actuated signal control [...] inductive loop sensors embedded into the road surfaceIt blows my mind how many people don't realize that's what those circles near the stop bar of an intersection are for, and will then stop 3+ car lengths behind the stop bar, and then can't figure out why the light is never going green.> two signals closely spaced…
2 more of the top 3 · 5 posts in this stretch
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There's a major arterial near my house. One day, the power went out in the area, and the traffic lights went dead. I discovered to my amusement that throughput on the arterial and its crossings increased dramatically, as drivers simply cooperated with each other.There are already cameras on top of all the lights. How hard can it be to hook them up…
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The two-adjacent signals causing extra traffic is something I encounter daily in Berkeley -- Ashby / Claremont & Ashby / Domingo. I probably blow an average of 5 minutes a day cause of that double intersection.To highlight one other point from the video: You want a light to be able to clear the line built up each time. If not, flux in > flux out…
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HN user at1as submits 2019 traffic-signal explainer, reaches front page
The user at1as posted the 2019 Practical Engineering article on how traffic signals work to Hacker News, where it reached a score of 57 with 36 comments; the same submission was mirrored by an HN Frontpage RSS bot account.
“Controlling the flow of traffic through an intersection, otherwise known as assigning right-of-way is an enormous challenge and almost always requires a compromise of numerous conflicting considerations…”
— Practical Engineering, article author · source -
1 outlet How do Traffic Signals Work (2019)
first by HN Frontpage, 2d ago
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What people are saying 14 voices from 1 site · best of 20 · verbatim
- Sep 22
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Every time I see an intersection fallback to 4 way stops, traffic slows to a crawl. Roundabouts are the true answer.
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I think this would depend on the intersection.If the lights are at an intersection where a very busy road crosses with a less busy road then turning it into a four way stop just turns it into a traffic jam on the very busy road. I’ve seen this happen during rush hour in the United States multiple times.When it’s not rush hour though maybe the…
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Where I live they've been actively removing traffic lights. One strategy is to turn multi-lane roads into single-lane ones with a lower speed limit, and crossing islands for cyclists and pedestrians. Cars move slower, but more consistently, traffic is better able to self-regulate, so while I haven't timed it, I'm fairly sure traffic is actually…
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I see so many places that could have roundabouts instead of lights. Plus, if the maximum speed inside my town was lowered to 40km/h, we could have shorter yellows. You can never reach the 60km/h limit for more than a few seconds anyway.
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Here's a surprising anecdote I've discovered living in South Africa wrt traffic lights.When they're not working at a busy intersection in my area, the intersection effectively becomes a four way stop. And I've found that as long as everybody follows the rules, traffic actually seems to flow FASTER than when the traffic lights are actually…
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> You want a light to be able to clear the line built up each timeIdeally, yes, practically, no. In reality you also have to consider what comes downstream of a traffic light. It doesn't help at all if one traffic light has perfect throughput only to put everything into gridlock in the next node.That means in most real traffic systems each…
- Sep 21
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I live in a small city in rural area (Nevada City/Nevada County, ca).Traffic lights here are unbearable. The situation, as I understand it, is that there's tradeoff between through-put and latency. Basically, if you make each cycle of a light really long, like 1-2 minutes, then how many cars get through, can be maximize. Sounds good but such…
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Reminds me a bit of the MIT study that relates to spacing in densely packed roadways [1]. Whenever I apply the rule (look into my rear view mirror and decide how late to slow down), I feel like a good little molecule in an ideal gas.[1]
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I live in BC near a highway intersection and in my experience they seem to do what you suggest.I approach from my street and if the highway is empty, almost instant green. Otherwise I have to wait some time until the road clears. During peak hours when the road doesn’t clear, it just seems to go green after a fixed time.
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One thing that I noticed about traffic lights in CA -- there will be like 4-5 cars who are driving on the major road and approaching the traffic light. Then a car arrived from the small road at the intersection. The traffic light will then turn red for cars on the major road, so all 4-5 cars will brake and stop at the light. The small car proceed…
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They are very common all down the SF Peninsula because of the Caltrain tracks. There's California/Carolan all through Burlingame (which has some of the most dangerous train crossings in the country because you can get stuck on the tracks between the lights), and then it follows El Camino on one side while the other one goes from Delaware (San…
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>As sensors become more ubiquitous and computing power increases, traffic management may slowly but surely be relegated from civil engineers to software developers and data scientists. But, that also means that ASCT systems may be more vulnerable to security threats, a scary thought if they’re controlling the signals for an entire city.I feel like…
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I always thought that traffic signals should be trying to minimize squared anger, where anger increases linearly with time spent waiting at intersection.
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I highly recommend Traffic Light Doctor on YT, if you're into this sort of thing.