How Traffic Lights Work

a little listener mail. This listener mail comes from Nate. He says, Hi, I'm a teenage listener from Colorado and just received my learners permit. I'm often frustrated by stoplights that are too long or short for the intersection. That got me wondering about how these things actually work. So I'm hope thing that you can do a podcast on how stop lights work. Thanks from your best listener in Colorado, Nate. Alright, Nate, we're gonna talk about some some traffic lights. Yes, and

uh do you like traffic lights? I like traffic lights only when they're green. Apparently Nate does. Alright, so um, well, let's you know we all know what traffic lights are. I don't think we need to do the breakdown that we usually do. Yes, Uh, It's it's funny because when this email came in, I started thinking, Um, you know

how very simple this idea is. You know, you've got a box and it's got some lights in it, and they have very red light or yellow light and green light at least here in America, and they switched from one another. Well that's pretty simple. Then I thought, well, you know, there really is a lot more to it than that, because it's not just the lights, it's also

the controller in the whole system that goes behind it. Yeah, especially when you look at it from a macro level like a city, a city size level, it becomes unbelievably complex because you're not just talking about one intersection. You're talking about traffic patterns that go across an entire city. Well, yeah, and you can use uh, if you've got the system that can handle it, you can use uh, you know, a whole computer network of traffic lights to uh to

arrange the way traffic would flow. That is, assuming everybody is obeying the law right. Well, and and that's important to remember because you know, in the old days, making adjustments to traffic lights was pretty difficult, I mean, or at least it was more involved than what it is today. Today. You know, there are fully automated systems where you can walk into a room and see the redoubts and kind of get a real life, real time picture of what's

going on. And if traffic is really bad in one part of town for one reason or another, you can actually make adjustments on the fly in many systems and and alleviate some of that traffic snarl. And and that's and it's crazy the sort of things that cause traffic jams. I've written about how traffic works for the site article. It's and it's really things that you would never expect to cause traffic jams can in fact cause them. And

sometimes it's just poorly timed intersections. You know, you could have two intersections that are even a mile and a half apart, but because of the way they're timed, traffic starts to back up. So we're gonna talk a little bit about what goes into making these lights work. I wanted to to step back a little bit and talk about the the older systems because some some towns in

in the United states still use these. Before the solid state electronics controllers for traffic lights became really the thing, we had mechanical controllers. Now, the lights themselves are electrical, and they're also modular in most cases, so that you can build a larger set of lights using the same basic units, so that lets you do things like create the you know, the turn signal lights versus the straight up and down you know, red yellow green lights that

we we know and love. Um Now, the old lights were halogen lamp lights, but usually between fifty watts and a hundred and fifty wats, depending on the size of the light. I would I would have thought that they would be incandescent, did you I would have you would have Apparently I would be wrong. Well, the current ones

are LEDs. Yes, they're much more efficient, and you can tell those apart two because it looks like they're a batch of little tiny lights in the circle where an older one of the older lights would be with the giant limbs. Right, So these older very cool that they're doing that because there's so much more energy efficient. No, yes, they're energy efficient, and you don't have to replace them

nearly as often the old ones. You would actually have to replace almost on a monthly basis, which of course could cause even more problems with traffic because it means that someone has to guts to go out there. And yeah, so these older MCCA Nicole controllers, they actually if you were to open up one of these boxes, here's where we have our standard, do not open a traffic control box. It's a bad idea, but probably against the law if you're not exactly Oh well, if you're authorized to do it,

then yes, by all these please yes. But inside these mechanical controllers, you would find a series of switches, uh. And at the base of each switch would be a little wheel. Now the wheel would have uh, twelve sections. Think of it like pi. So it's got essentially a twelve twelve areas sliced and one slice has been removed. When that slice comes round towards the switch, the switch is actually able to move out and the the switch

uh completes a circuit. All right, So there's a circuit for every single light at the intersection, for the red light, for the yellow light, green light, for each of the directions that are involved. Okay, this is a four wast yes, so four away stop. You know you've gotta you've gotta switch for every single light. Uh So these these the series of wheels are on a rod and they turn

um in time with a solenoid. The solenoid activates, it turns the wheels and then that's what actually completes the circuits and make the different lights switch from red to green to yellow. Makes sense. Um, these solenoids on a ratchet, so it can move down, but it can't move back up right, So it's not it doesn't justn't go from say, yellow to green. It has to go from green to

yellow exactly. And you also don't have to worry about it switching from green to red and then back to green immediately, because that would happen if if if it didn't have the ratchet, if it didn't have that breake stop there. Um, a light could change and then change right back, and then you've got traffic accidents all over the place. Uh So we got enough of those with it work properly. Right. So, the solenoid is connected to a motorized timer, and the timer is this little round drum.

It's got a hundred grooves carved into the side of this drum. Right within those grooves, you can place these little metal tabs, and the metal tabs also hit a little switch. When the tabs hit the switch, that's what activates the solenoid. Okay, so the drum is attached to a gear. The gear turns at a certain frequency, certain number of revolutions per minted. That's what dictates how fast

the drum turns. That's what dictates how how often the solenoid is activated, which in turn dictates how the little wheels connected to the circuits turn. That is the timing mechanism on the old UH traffic lights. And if you were actually standing next to a controller box and you were listening, you could probably hear the clicks as those

wheels turned into place. Um and UH. You could even install multiple drms within a single control box, so you could have it switch at different times of the day. So let's say during rush hour traffic you need one section of the road to be green more often than not. But then in the middle of the day the traffic patterns change, so you would want the light behavior to

change as well, and then maybe overnight it changes again. Um. You could actually do that by having multiple gears in there and have it switch automatically from one to the other based upon the time of day. Today, all of that is kind of it's it's antiquated. I mean, you don't find that in most systems today because now we have solid state electronics. Everything's on circuit boards. We don't have to worry about all those uh those actual mechanical

switches right right, But some of them are still on timers. Yeah, that's true. That's true. You know when you describe things that run on gears like that, I'm trying to picture exactly how this hardware goes together. I can't help singing Powerhouse in my head when I imagine the gears turning right. Uh. That okay, so and and Marshall actually wrote a blog post about this where he links to a video where

you can see. If you want to go to the to the blogs how Stuff Works and search marshall brains blog posts, you can find a post that links back to a video that shows these elements, these mechanical elements. Yep, yep, I remember that one. Actually I saw that as I was doing some some research. Um. Now, of course, uh, you know there are timers, and there are the uh, the lights that work on a sensor system as well.

Um and the timers seem like they're fairly self explanatory. Yeah, it's it's a state timer instead of mechanical same same philosophy as the mechanical ones. But yeah, different different methodology, but same effect. And that, Um, what happens. You've got some civil engineers who who observed the traffic patterns, who come up with the the pattern that should be used for any particular intersection, the amount of time that each lane of traffic should have to be able to pass

through that intersection. And um, you know, normally they will visit an intersection multiple times throughout different days to really get a good idea of what the traffic patterns are, because, like I said, they changed throughout the day, right, So in the evening it may be that, you know, one particular road is hardly a used all while another one is is really heavily used. You would want, of course,

the traffic light to favor the heavy use one. Um. And again this is it sounds kind of simple, except that when you take into account that you need to also look at the intersections that are essentially directly adjacent to that first intersection, like go in in every direction of that intersection to the next light and see what that timing is, because that's gonna affect the traffic as well. Well. Then you have to go one further out from those, and before long you're looking at a huge net that

incorporates the entire area, which is why it gets so complicated. Um, but not all of them are timers. Some of them use sensors, and these are the ones that I find fascinating. You can you can tell whether or not they have a sensor generally by looking at the uh not up where the traffic lights are, but down on the road. Yeah. Um,

there are different kinds of sensors. There are some intersections they'll use light based sensors like lasers or well, but but most, in fact, I would I would go so far as to say the majority of all sensor sensors

used in traffic lights are inductive loop sensors. Yes, that's the ones that you see, uh, the little grooves in the pavement directly underneath, like the white stripe where you're supposed to stop your car generally or or immediately before in some cases immediately after, which I think is kind of dangerous if that's the sensor that you're supposed to be on to trigger the light. But that has to do with the civil engineers, or perhaps they were painted

by uncivil engineers. Could be UM, but we wonder why is the Civil War even called that because they were not UM. So the sensors that are implanted underneath the roadway act very much like an electromagnet. Yeah, that's essentially what they are. They You've got a coil of wire that is underneath the pavement and m and what it's it's generating a magnetic field and when something really large and metal passes over it, it acts as an inductor. And when the system uh can detect that that increase

in the magnetic field. Because once you have the inductor there, that really boosts the magnetic field. Wants the system detects that, it's the indicator to say, hey, there's a car waiting at this intersection, so at the earliest opportunity we should look at switching the light so that the car can move. Um.

There's some problems with this system though, Yes. The big this problem being that if you are not in a fairly you know, like a medium to too larger sized vehicle, you may not have enough of metal in the mass of whatever vehicle you're in to to activate the inductor loop. So what you're saying is, if you were in our writing, say a motor scooter, motor scooter, bicycle, motorcycle, even even a small car like a smart car. Smart cars are tiny and may not even be enough of a mass

in order to get the inductor loop to activate. It may not generate enough of a magnetic field for the sensor to be tripped, which means you're sitting at a traffic light forever waiting for that thing to change, and it may take until another car or other vehicle pulls up behind you and perhaps if they're close enough, complete that that inductor loop so that you can actually trigger that light. UM and another or of Marshall's brains posts that that I saw when I was looking at UM

looking at some information for this podcast. UM. Some cities do have special infrared controllers too for emergency vehicles like fire engines and police cars, UM, that will allow them to trip the signals before they get there. It's it's essentially a remote control of sorts that allow them to UH to affect the signals and that that keeps them from having to of course, they're going to go to

the light anyway if they feel like it's safe. But if they can go ahead and change the traffic lights rather than having to go through the intersection without affecting those at all, that that helps them control the flow of traffic too to some extent. I have a hint for people who do drive motorcycle, scooters, bicycles, that kind of thing, if they want to activate those loops. Uh. This is not the same thing that Chris was just talking about. Chris was talking about actually changing the light

signals in an official way. Just activating yeah, to let to let essentially to let the system know, Hey, I'm here and I would very much like it if I could get across the street please. Um. What you can do is you can install a couple of magnets on the bottom of your vehicle. Uh. And by installed it just means stick because that's the one thing about magnets. They stick to metal. Uh some metal, so, yes, well

the metal that you typically find in vehicles. Yes. Uh. So you get a couple of powerful magnets and you attach them to the base of the vehicle, and then when you when you drive over the loop, the magnets in there, they usually generate a strong enough magnetic field that that will be enough to trip the sensor and let it know, hey, there's a vehicle here. So even if you're just writing in a little scooter that normally

wouldn't activate it, you'll get a signal. Now, these systems are connected, as we said earlier, to a master control system, and the master control system's job is to maintain an on everything that's going on across the the whole uh well, whatever the area is that it that they oversee, whether it's a local like thing or a citywide thing, um, but it usually it monitors traffic twenty four hours a day.

Often these systems can be manipulated remotely so that, let's say that that the the operator knows there's a fire uh a fire alarm that's gone off in a building downtown, and they know where the the fire engine is going to come from. They can actually proactively uh uh set up the system so that it stops traffic so that the fire engine can get through without any dangers of or reducing danger of collision. I can't ever say without any because there's always gonna be some something that falls

outside the extremes, right um. And they can also just keep an eye on how signals are behaving and if they if there's a signal oiled edge or anything like that, they can arrange for an electrician in or or you know, repair unit to go out there and take a look at it. Um. And they have to consider a lot of things when they're putting light. As we said, when they're putting these traffic lights in, it's everything from the

spacing between the traffic signals along the street. Um. They have to look at the different timing aspects of each of those. They have to look at just the traffic volume. How many lanes of traffic are there. Is it going to to handle the traffic volume well enough so that you don't have to worry about fiddling with the timer so much? Um, even the driver behavior on that street. Because of course, Chris and I know from living in Atlanta, there are different parts of Atlanta where people drive in

incredibly different ways. Let's say, like to eight five. If you're not going to eighty miles per hour, you've just

been run over. Now, granted to also does not have any traffic lights on it, but um, yeah, there's surface streets, Like there's certain areas of Atlanta where you know, people drive nice and leisurely and and everyone's kind of you know, laid back and relax in other parts of Atlanta where you feel that, you know, you've just entered the road warrior and two men will enter and one man will leave, and you know, Mel Gibson's right behind you and burying

it down. And Okay, I've had some really really weird dreams lately. As all I'm trying to say, I understand which so yeah, the driver behavior actually does play a part in how traffic managers look at at setting out

traffic lights and timing them out properly. Um. And it seems to me that that, um, if you've got lasers or inductive loops or uh you know, even those rubber air filled rubbered hoses I guess they're pneumatic hoses of some kind that uh tell the box that their cars waiting, it seems like including those would sort of wreck the plan of trying to manage traffic I mean other than I mean and by by a master control program. M.

Wait a minute, I'm beginning to understand everything and of line. Um, but no, yeah, having a master computerized program, uh, it seems like it wouldn't work. It wouldn't be compatible with having sensors at individual intersections that allowed cars to trip the lights as soon as they arrived and we're sitting there at the stoplight. It seems like it would be more it's more sort of a if you will, lazy fair version of traffic. I think they'll make the lights work.

I think part of it. I think part of this that when that detects that there's a car there, um, it doesn't necessarily say okay, well we need to change the lights now. It's it's saying, okay, we need to change the lights at some point when it's safe to do so. Keep in mind that if they're steady traffic going over an intersection, that and that inductor loop, actually it's picking it up because of course you're still having

cars going through. So these systems are are fairly intelligence probably the wrong word to use, but they're they're able to detect traffic at uh if it's if everything's working properly, they're able to detect traffic and a fairly rely I will rate. So it's still it's not as planned out as maybe the timed ones are in the sense of it doesn't take as much human intervention. But there's still you know, it's kind of it is kind of adjusting

traffic on the fly. But as I should point out that, I realized that as soon as you pull up, it wouldn't necessarily trigger the sensor and automatically turn the light, because if the box can tell that there are there's a steady flow of traffic going over the inductors in the other lanes perpendicular to the ones that you're in, um, obviously it's not gonna go, oh, well, hey, you know, I'm just gonna go ahead and stop them. This one guy really needs to go, so we're gonna stop everybody else.

I sort of always figured that it was a situation where they go, okay, well it's been ten seconds since the last car, it's been fifteen seconds since last car. Traffic is slowing down, there's less of it there. It's safe to go ahead and give them a yellow light

to prepare them for a red light. And some of them work on a on a combination of the timer and the sensor, so some of them have like even in some cases you'll you'll come up to traffic lights that you know have the sensors in them, and yet the traffic lights occasionally change on their own because that they do ultimately rely on a timer system. Um. And the anat thing about those timer systems you can actually program them so that they behave differently on different days

of the year. So if you know, for example, that say on fourth of July, that a particular stretch of road is going to be really heavily used, you can program the system ahead of time to compensate for that before you know, like you know, back in June you could you could program it in and then fourth of July comes around and the system behaves just as you told it to, so that everything's working properly. Um. Cool, Well,

I'm trafficked out. Well, I think it's a it's one of those things that we sort of take take for granted. But the the systems that they've put in to make this work, um, and try to be sort of intell gin about the way you manage traffic flow. I mean, it's obvious anytime there's been a bad storm and the and the lights are out, how much work actually goes

into planning out the traffic patterns of a city. And uh, the effect that having those sensors tripping the lights and and letting the boxes, the controller boxes know what's going on in an intersection. It's it's obvious how much difference that makes having that technology available to us, particularly in Atlanta, where people seem to forget that if the traffic light

is out, you treat it as a four way stop. Yeah, so, Or when they're flashing yellow on one side and you're supposed to proceed with caution versus flashing red, which means you're supposed to come to a complete stop, right and everybody stops regardless, or if it's flashing yellow, people don't slow down at all. They're just like that means go, that means actually go really fast. Yeah, if you're if you're a starman. Yeah, okay, I've got nothing for that.

If you got that reference right in at tech stuff at how stuff works dot com, or if you have any other questions for that matter. But speaking of other questions, I actually have another listener mail. This listener mail comes from Trevor, and Trevor says, Hey, Chris and Jonathan, I just finished listening to your podcast about cyborgs and cybernetics, and it made me think about a recent book i've read, Revelation Space by Alistair Reynolds is an excellent example of

advanced cybernetics. UH, pertinent throughout the book. I just thought I would write and give you that book. Keep up the good work, Trevor. PS. I know you constantly talk about virtual environments, but how come I've never heard anything from you about Second Life. I'm an avid member and I'm curious why you avoid it. Well, Trevor, it's not that we're actively avoiding Second Life, at least not more than anyone else's. UM. Second Life is a virtual environment.

It's kind of think of it like a massively multiplayer online role playing game without the game. Yes, I mean you can create games within Second Life, but there's no game element directly involved. There's no object of winning, there's nothing like that. It's not you know, but you do create an avatar and you can wander around a virtual world and own virtual property and interact with other people who are also in this world. UM. When it first premiered,

it actually made a big splash. You remember that, like all the companies that were getting into it. Yes, there were a lot of companies getting into it. There were a lot of other organizations trying to uh, trying to find ways that they could offer their services in the space. Uh. You know, and I know for a fact that some libraries were opening virtual reference desks in second life so that they could, you know, have people come in and ask them questions. There were bands that would have virtual

concerts and second life. Um. Yeah, you would find things like I think a lot of news agencies built like news desks in second life where you could you could wander into this virtual environment and kind of see what was going on in the real world. Uh. It was an idea of like places where people could have virtual meetings, where you could have maybe a conference online and have people show up in second life to attend the conference, and you wouldn't have to leave your home, right. A

lot of interesting ideas. It just never really took off to the It never lived up to the potential it had. Um, people still use it, but it's lots and lots and lots of people still use right, but nowhere near the number of people that everyone thought was going to use it.

Like a lot of the companies that went in and spent money developing this stuff have since long since abandoned those those stations within second life, just because they weren't you know, it didn't make sense to keep supporting something that was getting so little traffic. So that's really the reason why we don't talk about Second Life. It's not that we have anything personally against it. In fact, I've written an article about how Second Life works, but it's

just that it really didn't. It doesn't have the impact that uh, that it potentially could have, and so it just we just don't end up talking about it. But if you have any other questions, like I said, tech Stuff at how stuff rex dot com. And remember we have an official text stuff Twitter feed now yep, tech Stuff h s W. Yep, it's all together. So just go to Twitter and look up tech Stuff h s W. You'll be able to see our latest tweets, and you

can join our Facebook fan page yep, yep. And again tex Stuff hs W. You can find us there under that handle um and uh interact with us a little bit, you know. When we uh we published some some articles and things, you can give us some feedback there as well, right, yeah, and we really do like the audience interaction. It's it's becoming increasingly more important to us. So please if you have different ideas, you want to suggest something for the podcast.

Uh you think that maybe, for example, we're talking about the possibility now, this is a possibility of looking at h at some point interviewing other people on the podcast every now and then. If you have ideas of who we should talk to, give us a shout. Let us know. Remember we're pretty low on the grand scheme of things. So if you're saying Steve Jobs, keep dreaming, I want to interview Benjamin Franklin. Yeah, I had a lot to do with tech. I'm sending Adele Lovelace. Oh, should be

another good person interview. Yeah, all right, Well that wraps this discussion up. I hope you enjoyed it, and Chris and I will talk to you again really soon. If you're a tech stuff and be sure to check us out on Twitter Tech Stuff hs wsr handle and you can also find us on Facebook at Facebook dot com slash tech Stuff h s W for more on this and thousands of other topics. Does it how stuff works dot com And be sure to check out the new tech Stuff blog now on the House Stuff Works homepage.

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