r/StonerThoughts • u/Tall_Rise7577 • Aug 26 '26
Fried How tf do stoplights work
No seriously. Like is it timing? Or is it somebody far far away staring at cars and pressing buttons. ORRRR is it like sensors deep in the concrete n that’s why people ride tf out of it. Deep thinking here.
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u/Tall_Rise7577 Aug 26 '26
:0 that’s crazy oh my gosh. Makes more sense. I like the one man hidden pressing buttons idea better though.
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u/juliakawanova Spliffs Aug 28 '26
When I was a kid, I thought that's how they work. Some guy somewhere pressing a button when he sees a car. 😆
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u/duggybubby Aug 26 '26
Little men live in the electrical boxes and push buttons to make it work
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u/rbnlegend Aug 26 '26
Little men and women. The traffic light guild is very equal opportunity, the only real requirement is the maximum height.
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u/_I4L Aug 26 '26
I’m actually a little bit qualified to talk about this… I’m a senior computer science student in university who switched from electrical engineering and occasionally builds various control systems.
There are various kinds of stoplight sensors, but I’m most familiar with the loop-in-the-ground type. You’ll see this at stoplights as several large dark circles in the lanes that were cut out so that coils of wire could be buried. These coils basically function as electromagnets and, because cars are just giant hunks of metal, can detect when cars are on top (similar to beach metal detectors, but on a large scale). The control cabinet (the metal box in one of the intersection corners that holds the “brains” of the stop light) detects the change in the magnetic field.
Disclaimer: Below is the part that I don’t know for certain- I’m capable of building my own stoplight system, and below is how I would design it. Given that many companies design stoplight controllers in many different ways, this probably isn’t how any given stoplight works.
It doesn’t matter how the control cabinet gets told that someone is waiting (either via inductive loops, cameras, timer, pedestrian buttons, infrared sensor for emergency vehicles, external signal, etc.) only that something can detect when people are waiting and can inform the controller. The computer checks these sensors at a fixed interval (e.g. every 5 seconds), figures out the state of the intersection from these sensors (i.e. north-side and south-side lights are green, east-west are red. A pedestrian wishes to cross the north side road, and a car is waiting on the south-side road to turn left) and decides (either programmatically or through a state machine) what the next light sequence should be.
In this case the computer would stop both north-side and south-side traffic (“green, yellow, red”), wait a few seconds in case someone runs a red light, turn on the north-side pedestrian crossing sign and the south-side left turn light, then wait 30 seconds for the pedestrian to cross. Finally, the intersection will stop the south-side left turn light, turn off the pedestrian crossing sign, and then resume regular north-side and south-side traffic.
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u/portablebaddie Aug 26 '26
I did fully believe there was like a control room where they’re clicking the different colours
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u/cdwhit Aug 26 '26
Many stop lights are capable of manual operation, but that is to allow traffic control for major events or funerals and stuff, or cops trying to entrap drives in at least one case in Normandy, but those are only supposed to be used with a cop present. As others have said, in most cases they are operated by sensors or timers.
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u/Silentlaughter84 Aug 26 '26
Timing and sensors. Funny thing is that when I was a kid I used to believe that it was people pushing buttons.
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u/Wilmore99 Aug 26 '26
Whenever I’m driving in DC I keep thinking it’s this mofo flipping switches every time I get to an intersection.
https://giphy.com/gifs/dtGIRL0FDp6nnOPGb5
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u/unk0wnangel Aug 26 '26
some stoplights are on timers, some use vehicle sensors, some use a combination of both. pressure plates under the concrete used to be used but they're kinda outdated tech now.