Intro To The North American Traffic Signal

Traffic lights are so ubiquitous that we hardly give them a second thought, except to curse their existence when they impede us on our daily drive. But no matter how much it seems like traffic lights have the ability to read our minds and tell when we’re running late, they’re really not much more than a set of lights and a programmable controller. Simple in practice, but as usual, the devil is in the details, and for a system that needs to work as close to 100% of the time as possible, the details are important. Let’s explore the inner workings of traffic signals.

Electromechanical Timing

The traffic lights and crosswalk signals at an intersection are only the public user interface, of course. The interesting stuff is going on in the control box. There’s at least one at every intersection, usually a plain metal cabinet set back from the road, sometimes camouflaged with public bills or graffiti. But inside are the guts of what makes an intersection work and keeps vehicle and foot traffic moving smoothly and safely.

Unsurprisingly, most traffic signal controls started out as purely electromechanical devices. Cabinets were chock full of synchronous motors turning timing wheels with cams to cycle the intersection’s lights through the proper sequence. One old time controller that was common up until recently was made by Econolite, and the insides are a paragon of sturdy design.

The controller shown in the video is a pretty simple one for a straightforward intersection that only needs to run a fixed program or two plus allow for a timed switch to a “flash only” mode after a certain time of night when traffic drops off. More complicated electromechanical controllers implemented crosswalk signals coordinated with the traffic signals, and provided for magnetometers or pressure switches installed in the pavement to detect the presence of vehicles and alter the timing of the signals.

A System of Interconnected Things

Electromechanical controllers served municipalities well for decades, and some intersections are likely still controlled by one of these simple but robust devices. As technology advanced and traffic engineers came up with more and more complicated intersections to support increased demand, the old electromechanical boxes were retired in favor of solid-state controls.

The first generation of solid-state controllers began hitting US streets in the mid-1970s and provided for more sophisticated control and better safety. Modern controllers have to implement a conflict monitor unit (CMU) to detect any failures of the program and resolve them safely. When all else fails, the CMU will switch the intersection to an all-flash fail safe mode so that all traffic has to stop.

Leaving a major intersection on all-flash is obviously not something municipalities want to go on for very long, so networked controllers came along that could phone home for help. Networked controllers also offer the potential for coordinated control, where a central control system capable of monitoring sensors from multiple intersections can override the program at any intersection. Coordinated control has the potential for greatly increasing the carrying capacity of roads, but it’s not without its costs; like any other networked system, coordinated control systems are targets for attack.

There’s much, much more to the story of how traffic systems are automated, and our own [Bob Baddeley] will be taking a deeper dive in his article later this week. But before this introduction closes, let’s talk about preemption.


An Opticom preemption receiver and notifier light. Source: Wikimedia Commons

One interesting feature seen in some modern intersections is emergency vehicle preemption. These systems use a sensor to detect an approaching emergency vehicle and switch the intersection to all-red except for the direction the vehicle is approaching from. Most preemption systems in the US seem to use optical triggers; the emergency vehicle will have a front-mounted IR transmitter, while the intersection will have a receiver mounted near the traffic lights. The vehicle transmits a modulated signal to the receiver, which signals the controller to preempt the normal intersection program. Newer preemption controls use GPS to locate a vehicle relative to an intersection to determine if preemption is required.

Can I Do That?

No matter what color hacker hat you wear, thoughts can’t help but turn dark with the possibilities presented by preemption IR receivers dangling temptingly over busy intersections. Is it possible to zap an intersection into preemption with something as simple as a programmable IR remote? There are a ton of videos out there where teenage doofuses purport to do just that, and while we’re not saying it’s impossible, color us skeptical. Most videos show a programmable remote or smartphone with a Morse code flashlight app being used at intersections that clearly aren’t preemption-equipped. Dubious at best.

IR preemption signals from an approaching emergency vehicle could plausibly be captured by a digital camera and potentially be decoded for later playback. But there are a few reasons this is unlikely and not widespread. Preemptions are generally reported to a central station and would no doubt raise suspicions if one occurred where no emergency vehicle is expected. Anyone misusing the system would face federal charges of tampering with a traffic signal. And these systems do fail-safe, stopping all but one direction of traffic rather than actually allowing an attacker more control.

So next time you’re fuming as you’re stuck at an eternally red light, just relax and think about the how it’s all controlled — and pray for a fire truck to come up behind you and change the light legally.

Read more from this series:
Field Guide to North American Traffic Signal

78 thoughts on “Intro To The North American Traffic Signal

  1. Some mention should be made of the early networking of electromechanical traffic signals to provide a continuous stream of traffic at a certain speed (if you hit the sequence correctly and didn’t get slowed down) that could take you miles through a city without a stop.

    1. I remember that very clearly. In my town (Eindhoven, The Netherlands) they had special traffic lights at the side of roads with numbers instead of colours; ie. 30, 40 and 50. And if you kept to that speed, you did not have to stop at the next crossing. Worked like a treat. The system was called Groene Golf (“Green Wave” in English).

      1. I’ve heard of this also. In Lincoln Nebraska, on a certain street downtown, you used to be able to go 36mph and get all green lights.

        I knew of a guy who thought if it works at 36mph, it should work at 72mph. He tried it and was even driving up on the curb to maintain that speed. He was pulled over and nearly went to jail. He got out of trouble by agreeing to leave town for a few years, which he actually did.

        1. In Portland, Oregon in the 60’s (about 65) all the lights on Stark Street from about 60th to Sandy Blvd were synchronized to 30 MPH. If you drove 30 (even in traffic), you would go through all the lights without stopping. That was about 55 blocks. Now you can’t go more than an 1/8th mile and not get a red light at any speed. They are “smart” lights :-)

        2. I would not expect going at twice the posted speed works. If you wish to ride the first harmonic of the “green wave” you would actually have to go half the posted speed, i.e. 18mph.

      1. Likely a timing signal that ran by cable through the system, using the “electro” rather than the “mechanical” part of that. School clocks had syncro systems for many years before computers too … sometimes they even worked.

        1. Railway station clocks also synchronize electrically to a master clock. But in one building of technical university I wondered about a series of narrow sheet metal covers (about 1m or 1,5m long and like 15cm wide) on the wall, advancing from the clock in the main stairwell for several 10s of meters even with some 90° bends to another clock. Finally I found one open cover and surprise: There was a driveshaft and angle gears behind. This was in the late 90ties. But I don’t know if this was still in operation or if the clock was retrofitted with some more modern system.

      2. There is a dwell key on the timing wheel where it will stop and wait for a 120volt pulse from the local master controller. The timer will then move past this point and perform another cycle. (Main Street green through side street) offsets ( time space interval determinations between signal) are predetermined and the dwell keys derived from them.

  2. This is terrific, thanks! (old technology is always fun)

    A few quick bits of info for Bob Badeley for his article:

    1) there was an article published 25-30 years ago on how the NYC traffic-light system was automated – I read it back then, but haven’t been able to find a copy since. Lots of single-board Unix computers at intersections, and such, running a realtime OS derived from the (CTRON?) project.

    2) there was at least one paper written about a “computerized traffic circle” in central NJ – 1960s/70s? – it was either “Ellisburg Circle” or “Racetrack Circle”. The circle had really bad traffic that was improved immensely by the addition of a controller sitting inside a trailer in the circle.

    (sorry I don’t have more detail about those)

  3. I remember, while in college in Springfield Ohio in 1972, that they had a system for detecting emergency vehicles and changing the lights. It would be interesting to know how these “old” systems worked.

  4. I remember that episode of MacGyver where he has to flee the city with the Gypsies and he stuck business cards or something in the wheels of a light controller like that to tie up the government guy’s vehicles in traffic.

    I think that episode also used a bunch of recycled scenes from the original Italian Job too as a point of trivia.

  5. Related tidbit: if you press the button at a pedestrian crossing, chances are it won’t actually do anything. At many busy crossings they are simply disabled. Studies have shown we tend to obey traffic lights if there’s a button. Here in my neck of the woods pedestrian and cycle lights are now fitted with LED ring displays that “count down” the time until you can cross.

    1. We have the countdown timers on the pedestrian signals her as well. It is infuriating when the countdown gets to zero, goes blank, and then nothing happens. Signal still doesn’t change for tens of seconds (which is like decades when you’re waiting on the stupid light).

      1. We also have odd traffic lights where communication between them is slow, meaning that looking at your closest light at a complex intersection isn’t the best tactic (as a pedestrian mind you). It’s extremely irritating as they all switch to red at the same time so some have severely shortened green-times.

      2. The steady “Don’t Walk” after the flashing “Don’t Walk” of a pedestrian phase of a light is a carefully calculated decision. Typically when it is longer than you expect, it is used because pedestrians hold up turning traffic in such a way that it would (or did) impact traffic flow.

        There are a lot of reasons why it could happen. One is unpredictable side road traffic where the primary road’s portion of the cycle is much longer. In cases like that, the light prepares to change without a car being present because if a car arrives they don’t want to wait an entire cycle or more.

        Another case is an intersection where one direction’s traffic frequently makes turns without a dedicated turning lane. There options to help keep traffic moving include increasing that road’s portion of the light cycle, adding an “advanced green” phase, closing that section of the intersection to pedestrians, and of course as you’ve seen, shortening the Walk time and increasing the Steady Don’t Walk time.

        All in all, when you see a deviation from the “typical” flow of an intersection, it’s designed to be that way to solve or prevent a problem.

  6. Some of the pre-emption systems in the early 90’s (when I was in high-school and up to no good) could be spoofed by flicking your highbeams at the approximate frequency of the lights on an emergency vehicle (kinda like old-time phreaking, you just had to get the right frequency)….

  7. Mid 90’s I worked for a small pc shop and went onsite to fix a couple PC’s for the city where they did the light maintenance. They told me they test every light controller for 1000 cycles and every time 1 cycle failed. They had both the vendor for the controller and the vendor for the test system and neither could explain why one test out of 1000 failed.

    I have personally seen lights fail twice, but thats since the late 80’s so almost 30 years and over 300,000 miles.
    One all 4 ways turned green for a split second then started flashing red. The other just got stuck on red all w ways for a long time until people behind the first person in line started honking and then the first person in line ran the light and was pulled over by an unmarked police car.

      1. Decades ago I came upon a failed traffic light (all dark). I got out, walked to the middle of the intersection and started directing traffic until a cop came and took over. MOST people were willing to accept my “commands” rather than sit there wondering what to do… because traffic was flowing more efficiently. I was “fair” with all the cars, giving the largest backups more time, but didn’t let people sit in turn lanes indefinitely.

    1. If they’re stuck on red, then that could be a preemption system, and an emergency vehicle might be approaching. Usually a fault will put the system into flashing reds, which indicates that the signal has now become a 4-way stop.

    1. Yup, it’s the “Hello World” of HDL. I’ve done it in Verilog, VHDL, and on an Allen Bradley PLC in ladder logic, at least. Probably more.

      I was much more interested in the mechanical timer, looks like a close sibling of the the Intermatic light timers for turning your Christmas tree on and off.

      I rode with a guy who had an IR flashlight with a small micro in it to modulate the emitters for those pre-empters. At the time, it was just a fixed frequency, pretty low like in the Hz range. It was later changed to an encoded version but you could probably capture that, too. Once it was changed to a Federal offense, it was less fun.

      1. ” It was later changed to an encoded version”
        if there was more than one emergency vehicle approaching an intersection, some gave priority to a fire truck (harder to stop) over an ambulance or police car,

        1. Also for priority- usually police are prioritized to get some kind of ‘authority’ on-scene soon as possible, then fire trucks because they can serve a dual-purpose and act as EMS in addition to normal duties, then ambulances for specialized medical care.

          1. In Canada it’s fire- ambulance – then police
            I’ve heard that in Britain technically by law it’s mail – fire – ambulance- police – because nothing is more important than the Royal Mail.
            Think- get people out of the bad situation (fire) then temptress them medically (ambulance) and set up crowd control when possible (police)
            Though really fire fighters like to let the cops run up to the situation first sometimes as they make good canaries

    2. I had to do one as a class project, the most interesting part being that after adding in the countdown displays, the lights appeared to change faster even though they didn’t! Would be nice if real traffic lights had countdown displays. (Actually, in some parts of China, the traffic lights really do have countdown displays!)

      1. Bad idea here, mostly due to drivers habitually speeding up on a yellow light. If they can see how much time they have to beat a light, they might be more likely to speed up sooner, and this would inevitably lead to more accidents, like rear ending cars who come to a stop because they would rather not get to beat a light.

  8. Pre-emption in our state (California) is not done with IR per say; it is a precisely timed strobe in the light bar of the emergency vehicle (only ALS ambulances and fire trucks in most places) most often covered with a VLF (visible light filter). I won’t mention the specific frequency outright, but Google knows all.

    I say this all to de-troll my next comment: all the claims of headlights, phone light apps, ect. triggering pre-emption are BS!

    I have installed, serviced and tested emitters for years. I can even hear from the clicking when they are starting to fail and the timing circuit is off. The cycling is at a speed that is not at all easy to create manually. And headlights aren’t meant to/ can’t cycle that fast, and a phone strobe is nowhere near bright enough. We watched a friend try for hours waving a flashlight at a reciever outside his house, and only told him the problem when the beer was gone. Good times.

    1. There was one traffic signal on the south west side of Tucson that if you flashed your high beams at it (I only tried it at night), it would trigger the green light in your favor. That was in the mid / late 80’s. But that seemed to be the only one in town that behaved that way. Certain areas around Tucson (back then anyway) was also a test area for new highway tech.

    2. I always thought the “flash your high beams to turn the light green” story was nonsense until I tried it one day at an empty intersection with no one around and it worked. I’m sure newer sensors are far more restrictive but flashing high beams tripped the sensor on this particular traffic light.

  9. Here in Hotlanta, there are few streets – not talking about interstates – where people in the NW section can cross the Chattahoochee river to go into Atlanta. One road – Johnson’s Ferry – at the Cobb/DeKalb county line had a rather notorious signal that favored cars coming from the homes owned by the 1 percenters, and would snarl up the traffic coming from the north.

    Supposedly, Cobb county tried everything to get the signal changed so the traffic flowing southbound would move better, to no avail.

    One weekend, someone put a rather large caliber slug into the traffic control box. This necessitated manual police control at the intersection and traffic moved MUCH better. When they replaced the box with a heavily armored one, the timing was changed so it did not favor the subdivision traffic. The entire mess ran MUCH smoother.

    1. I lived about 10 miles from that intersection. It was in the papers and news for a couple of day. Cobb had six lanes to the light and Fulton narrowed it to two lanes right after the light. The light was originally at 2 minutes of green for the Cobb side as everyone went to the city to work. Fulton had the genius idea of reducing that to about 50 seconds of green creating daily hour plus long backups for the last mile out of Cobb. After the box was shot, Fulton graciously added 10 seconds to the light reducing the traffic jam to about 50 minutes for a mile. Fulton widened the road to four lanes on their side quite a bit later. Traffic still sucks though.

      1. Studies have found that adding more roads/lanes does little to relieve traffic congestion. The easier they make it to drive a certain route, more people use that route until it gets congested again.

  10. When I was in the US I nearly came a cropper on a couple of times trying to cross the road. I remember somebody explaining something about cars being able to turn right at some junctions even though the light was on red, I never really got used to it.

    1. You can’t turn right on red in your area?

      Here you can except for certain lights that have signs saying you can’t. You do have to stop, you just don’t have to stop and wait for the light. Just stop and look for traffic. It’s just like a stoplight.

      Does this seem strange?

      Why wouldn’t one be able to turn right on red? When turning right (in a country where one drives on the right) you are not crossing against traffic, only merging into traffic (if there is any). When going straight or turning left you are crossing traffic. That is different.

      1. I’m not sure if it has changed, but Right on Red was more common in the western States, where there was less traffic.
        I admit that “double Right on Red” is a spooky thing for me.

        1. Right on Red is legal unless otherwise posted in most eastern states as well. In the US there’s also a funny thing where it is legal to turn Left on Red if you are turning on to a one-way street.

  11. Amusing to compare it to Sydney, Australia.

    In the 70’s in Australia the Road and Transport Authority – back when government departments were expected to be innovative and actually develop things – they developed SCATS. This is a city-wide coordinated, computerized traffic management system where each traffic light was intelligent and sensed traffic passing and waiting:

    SCATS is now used (Wikipedia) 154 cities in 25 countries.

    The systems can run autonomously, but normal mode is networked (via a variety of L1 technologies). They also even have hardware interlocks, preventing hacking or computer failure from creating dangerous light patterns (everyone green!) The specification itself is called TSC/4.

    I am told that the original 70s iteration for each light had a LSI-11 controlling it, but I cannot verify that this is true. Nowadays multiple manufacturers product TSC/4-compliant hardware, which is installed in overly large roadside cabinets, which originally accommodated the old hardware.

    In these sad days, it’s inconceivable that some manager in the Road and Maritime Service (RMS), the descendant organization for the Roads and Traffic Authority (RTA) which developed SCAT – seeing a need and actually making the commitment to R&D and deploy of something like SCAT. They’d just go to tender, and if nothing was available, they’d do nothing.

  12. Just got one of the main T-intersections (its actually a 4 way but one sides comes from a parking lot that doesnt get very much traffic) in my small town replaced after being out of commission for the past couple of months. It initially defaulted to a red/yellow before they got permission from the state (the top of the “T” is a state road) to change it to a 4-way red.

  13. In ’94 or so, I was stationed in Montgomery and I was going to school in the afternoons after work. One of my classmates worked for the city. At the time I was a huge fan of OS/2 and the geek talk went on and on. As it turned out, the traffic light system in Montgomery ran on OS/2. I wish I knew how long it remained.

  14. I was a firefighter in Anchorage Alaska from 1975 – 1990. In the early 80’s we tried using a system called “OptiCom” which would trigger the lights using a forward mounted strobe with a rate of about 4 pulses a second. It was dropped after about two years. Electronic hackers soon had strobes on their cars and apparently the sun would gleam off windows and water and inadvertently trigger it.

  15. I was an EVT (Emergency Vehicle Tech) for many years and have installed preemption systems, there were 2 types in use at that time (2010), Infrared and Strobe. Both used a specific series of timed flashes coded to activate the system unique to that region or municipality and would only work in that town.

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