Hello hello

Work as a technician fixing traffic lights (electrician). From electronic fault finding to replacing posts that have been hit.

Almost everyone I meet new asks me something about lights. How they work, how to make then turn green faster. Switch off speed cameras etc.

Thought I'd share. AMA!

Edit: busy with work and such, delayed responses.

I intend to edit and write a faq.

Comments: 3011 • Responses: 41  • Date: 

mtlltm1481 karma

Hi, how accurate is this xkcd? http://xkcd.com/277/

ash_wipe1333 karma

3rd window. Accurate.

For the rest, Lol

I too used to complain about the lights. People are impatient, can't keep everyone happy

MustangMark83447 karma

There's a traffic light where I live that between the hours of about 10 pm and 6 am, it will go red after only 15 seconds of being green, even when there is NO cross-traffic and I am on the major US highway. Then it proceeds to stay green for cross traffic for probably a good minute, nobody around but me, every night. Who designed shit like this?

It catches me 90% of the time coming home from work at night, and I just sit there, scratching my head, because there is NO ONE FUCKING AROUND BUT ME!!!!!!!!!!!!!!!!!!

Jms_1989773 karma

Not to steal any thunder from OP but I'll happily answer. There are many types of detectors in intersections... Inductive loops, camera, infrared, ped push button, sensys wireless "pucks". All sorts of things. Anyways, these detectors feed to input cards and racks that then send a signal to the traffic controller placing a "call" telling the controller to service that direction. In the event that the detector fails in some way the card in the detector rack will default to always show a call. Imagine a broken detector and you pull up to an intersection never to get a green light. So instead, the card will make that direction alway get a green every cycle whether a vehicle is present or not. So the likely scenario is one of four things. One a detector is damage. Two the pedestrian push button is damaged or stuck on, three the light is unactuated(timed with no detector), or four the city is a bunch of idiots and won't allow the tech to service it properly. Hope that helps haha.

ash_wipe499 karma

Bingo. We have a winner

jeffsnotfaking873 karma

The little buttons that pedestrians press when they need to cross the street... Do those really work?

ash_wipe1209 karma


I have no idea how word got around that they don't. Recently on the radio a guy called up saying they're a placebo. Haha

jeffsnotfaking462 karma

Does it make the light change quicker? What about if you press it repeatedly?

ash_wipe839 karma

No, it'll introduce that pedestrian movement.

Pressing it again doesn't make it come up quicker.

fulfilledprophesy527 karma

Is there a secret code that we can press to make it change faster?

Bash - bash - bash - pause 5 seconds - bash - bash bash - bash bash bash.

If not, can we implement one?

ash_wipe1731 karma

I wish. Haha

In Singapore I noticed swipe card readers on the pedestrian buttons. The Elderly / disabled get issued them, swipe at the lights and you get extra time to cross.

Bonus round!

aggemamme29 karma

Even at 'lone' pedestrian crossings with no other roads, there is usually a delay from you press the button until the pedestrian signal will turn green. The delay seems unnecessary, but why is it there?

To discourage kids from playing with it (they don't get satisfying immediate feedback that they can 'control traffic')?

ash_wipe109 karma

The phase just needs to run to the end of the sequence.

Ie, if it just turned green (the vehicles) and you press the pedestrian button. To go straight to amber and red again would be crazy.

praetorblue115 karma

Here in NYC, the vast majority of the buttons are non-functional.

Source: http://www.nytimes.com/2004/02/27/nyregion/27BUTT.html

Around here, lights change using a set pattern.

ash_wipe107 karma

Haven't clicked the link.

But lights also work in a set pattern here also. I believe they all do. Until AI takes over. Some one is still out there programming.

ampellang279 karma

Until AI takes over.

The year: 2045. Skynet has effectively exterminated the human race by encouraging them to cross the street at unsafe times...

ash_wipe72 karma


jeffinRTP27 karma

Are you saying that they change the timing of the lights or just turn on the walk/don't walk signal? If the lights are synchronized thought out the city how the traffic engineers would allow one person to change the sequence and timing.

ash_wipe76 karma

If the synchronisation is broken a few times (due to other demands such as pedestrian movements, opposing roads) the lights can break away from the chain.

This is why you can't always get a perfect green run. Ideally that's what the lights are designed for.

ash_wipe59 karma

The timing of the lights stay the same. The walk phase will be introduced when it is allowed to. After a set of sequences have been done. (there could be none or many)

reddit_alt_username0 karma

They definitely don't always work.

ash_wipe35 karma

I can assure you they do. (except in the odd case of an actual fault being present)

Just don't expect to press it and instantly get a green light.

WiggleBooks3 karma

Uhh, not always... In my city the buttons were disabled so they literally did nothing. Wasn't too bad though, the system just assumed that there were pedestrians that were gonna cross (this was in the downtown area).


"The push buttons were redundant, because pedestrians were always using them," Escobar said Friday. "There is a general premise there is always pedestrian activity (downtown) so there is no need for people to push buttons."

Winnipeg's public works department began disabling the crossing buttons four years ago. This means all the intersections in question have been set to display "walk" signals, regardless of whether anyone pushed the buttons, Escobar said.

The city did not publicize the fact the buttons were disabled to avoid confusing pedestrians. The buttons remain functional at intersections outside downtown, Escobar said.

ash_wipe5 karma

Like in the city, no one bothers pressing because they know it comes up every time the lights change.

But to say that the buttons do nothing and go to a different intersection completely different and just stand there expecting them to change for you is absurd. Is it that hard to press a button?

I see it all the time if I happen to be working the lights, someone will just stand there for 5 minutes and then yell out something to me. To which I'll walk over / tell them to press the button. To their surprise it works.

I think you'll find that your article is talking about a specific set of lights, (without reading) not all

GeneStarwind1025764 karma


ash_wipe953 karma

Friends are pulling your leg. Haha

100100111645 karma

What is your favorite dinner entree?

ash_wipe993 karma

Any form of Japanese food.

Ama, you're doing it right.

imsoupercereal559 karma

Who do I contact to get light timings reevaluated?

ash_wipe570 karma

Your local authority.

On the controller box there's usually a number located you can contact.

chudez558 karma

In the The Italian Job (the newer one) the"hacker" commandeered the control of the city lights to create a favorable traffic flow for their getaway. Possible or b.s.?

ash_wipe764 karma

The part where they make all the approaches go green? BS, bosses at work kept talking about the inaccuracy. -_-

chudez300 karma

BS cause the computers are hack proof or BS cause the technology doesn't support that kind of control?

ash_wipe695 karma

Sorry deleted. Wrote something that could've been misinterpreted. Haha

Bs because the signals controller would detect a conflict (causing a major fault) and go flashing yellow. Would be impossible to tell it to do that.

The memory that holds the conflict matrix (what can work with what) is non volatile. Stored locally on site.

dangerspeedman317 karma

This is so cool. I never thought I'd be this fascinated by traffic light facts.

Phycoz294 karma

You are now subscribed to Traffic Light Facts! Respond "red light" to stop receiving these messages!

BullshitBlocker309 karma

Green light. lets do this.

SkaveRat172 karma

In most european countries, there is a "red-yellow" signal shortly before it changes to green, to have a better response time when it changes to green and the traffic moves quicker

pHreaksYcle621 karma


ash_wipe739 karma

That's screen peeking, mate.


luke_in_the_sky77 karma

What if they hacked the box in each intersection to bypass the conflict matrix?

ash_wipe196 karma

Not possible. The conflict matrix is stored on a non volatile disk. Altering that would mean programming a new set of instructions.

Sure you could program a set of lights to do that. But that's different. Not hacking. It'll be doing what it's intended to do.

You'd have to design / program it. Write it on the disk. And insert it into the module / controller

DLaicH45 karma

Is it read from disk every time, or is it cached in volatile memory?

ash_wipe100 karma

Stored in RAM.

Referencing back every now and then.

JeremyR2231 karma

Bs because the signals controller would detect a conflict (causing a major fault) and go flashing yellow. Would be impossible to tell it to do that.

I've seen a 'major fault' that didn't cause flashing lights, though. It was a fairly new intersection too. Something went wrong and the whole system locked up - the minor cross-street was stuck on continuous green and the major road was stuck on continuous red. It was crazy - people were running the red in frustration, getting out in front of traffic that had the green and the whole intersection got completely jammed up. Once they got there, it took a couple of cops a while to get people moving again.

I was under the impression that wasn't meant to be able to happen? At that point, it would have been easier if they'd just switched themselves off. At least people would have known what to do about that...

ash_wipe75 karma

Not meant to happen.

I've attended similar faults. What I've been referring to as the controllers program is actually called the personality. That is what is telling the controller what to do and how.

A corrupt personality can cause that fault. And the controller may not pick it up.

Much like if your hard drive crashes in your pc and your screen is just locked up. You could also argue that the computer should have a fail safe to know that it's stopped spinning. But that set of operating instructions are on that hard drive. Uh oh.

All in all, still not a perfect world we live in.

reddit_alt_username458 karma

Why are there no traffic signal technicians in Atlanta?

ash_wipe429 karma

Cos they're all in Australia.

Thinc_Ng_Kap346 karma

In my city, we have signal changers at non major route intersections and some signal changers that are set back a few car lengths in the left tunr lane to trip the advance left turn signal to allow the extra cars that are turning to go quicker through the intersection.

There are always cut out marks in the pavement where these boxes are put in. This is where I put my car to trip the switch. What are they? Are they weight sensors? Are they some sort of electronic interference device?

ash_wipe546 karma


It's a loop of cable in a figure 8 formation. Where possible, it's laid under the road. Commonly they'll cut into the road for them.

It forms a magnetic field which picks up the metal in your car / bike.

ItsJaymay241 karma

Whenever there's an accident in my city (Adelaide, Australia), and an ambulance or fire truck goes through, it seems that every light is red, apart from the ones on their route. Can emergency services request a green lit route to get to an emergency faster somehow?

ash_wipe321 karma


I doubt they do it for every ambulance though.

tbird2490 karma

Doesn't this contradict what you said about the Italian Job scene not being possible where you can control the lights from a central location?

ash_wipe194 karma

Priority can be given to a phase.

Can't make all directions go green like in the movie.

snowagainst205 karma

Hello, I wanted to ask 2 questions.

Why did you chose this job?


What is the hardest part about your job?

ash_wipe330 karma

I did my electrical apprenticeship with this job. Prior to that I studied electronics & communications part time. Chose to do electrical as it's the hardest trade and wanted a challenge.

Hardest part, waking up. 6am starts. Haha

skunkylee151 karma


How do the automatic fault detection systems work inside traffic lights?

There was an accident where I live a couple of days ago where the red light bulb malfunctioned on the pedestrian crossing (didn't light up) and a kid got hit by a driver going through green. Sadly, the boy didn't make it.

I'm guessing the lights are connected to a network. For instance, how difficult would it be to place current detecting sensors for the power lines going to the bulbs and have them report a malfunction when there's no current flowing where there should be.

Barring that, what kinds of systems or safeguards are implemented?

Thanks for the AMA :)

ash_wipe148 karma

Where do you live out of curiosity?

A system like the one you described exists.

There are response times for faults. Ie, lamp outs. It may of just went out.

slightly_aroused108 karma

How do the sensors mounted above the traffic signal that detect a vehicle is present at the intersection work?

We have one at my office which works fine during the day, but does not detect cars at night... thoughts?

ash_wipe139 karma

There's a few types of motion sensor technology out there. Infrared for example. These sensors are no different to house alarm sensors etc. Just the application is varied.

Maybe the type of sensor used is a video type. Detects a change in the input image. Waits to confirm it is an input, (not just a bug flying by). Places input.

Maybe it's not detecting properly at night.


Is each light controlled by a control unit at its location, then networked to a central location, or is it only controlled by s central location?

ash_wipe87 karma


Each site is programmed. And running locally within it's set parameters as per the central networks command.

Ie, so you get a flow of traffic (like consecutive green lights)

HrRossi57 karma

Sometimes there are small boxes on the posts that look like speakers. What are those ? (usually on the top of the posts, with slitted front covers)

ash_wipe67 karma

I can't picture what you're describing? Where are you from?

It could very well be a speaker, for emergency broadcasts. We have speakers in the city (Sydney, Australia). But these are shaped like a mega phone.

tehWKD36 karma

Did your username came to existence when you were fixing a traffic light and angry Sean Connery drove past?

ash_wipe27 karma

Nah, highschool friends. Also have ash hole, ash tray. Punk ash.

fvdcsxaz35 karma

What is the process of determining a light schedule? Is it done manually, or are there automated programs into which you can input traffic data to find the optimal timing of red/green lights?

ash_wipe47 karma

Afaik each site is evaluated to determine the major road and minor road.

There's only a few guys doing them. They have their own set of rules to abide by. (to keep lights consistent, 4 sec amber time in most cases for eg). I believe their logic is based from experience

tkrynsky34 karma

There are a couple traffic lights in my neighborhood that you basically can't tell their color untill you're almost in the middle of the intersection - It's not that the light is dim or anything, it's some kind of filter on the lens. Do you know why something like that would be installed vs a regular light you can see from a distance?

ash_wipe48 karma

Is there another intersection close by? Could be so people at the near by one don't get confused.

Sometimes lights are angled down or louvres are installed for that reason

MrClubine27 karma

What kind of plc's do you use?

ash_wipe34 karma

Electronic ones. =p

We have 4 different types atm. 1 of which is new.

These are specifically made for traffic controller purposes. Not an off the shelf item.

DysprosiumJudas19 karma

What's the secret behind the pedestrian signals? It is usually whenever the left turners pass your crosswalk, but this is not 100% always reliable. Why are there sometimes extra left turns that you don't get to walk after?

ash_wipe37 karma

You mean when you're the pedestrian, right?

I think you'll find that'll only happen when there's cars / it's busy.

The lights will run through the programmed cycle clearing traffic in a pre determined priority order.

But you sometimes might get lucky and hit the pedestrian button at a time it's fine for it to come up.

Nb, when no cars around, (late at night for eg) it should come up almost straight away .

10010011118 karma

What type of voltages are you dealing with up there?

ash_wipe30 karma

230v. The lights themselves are 10v. (transformers)

aggemamme17 karma

Do you know of any traffic light systems that are 'online', i.e. could be controlled by a central "command centre"?

Some time ago I heard about GPRS (GSM) enabled signalling systems which could be dynamically altered from central control, in order to adapt to changing traffic patterns, etc.

In that case, have there ever been any vulnerabilities in such systems?

ash_wipe24 karma

I'm not sure what entirely you mean.

But all the lights here (Sydney, Aus) are connected to a network. If there's an accident or some road needs to be cleared priority can be put on that direction.

aggemamme14 karma

How's the wiring normally done? Is there one big relay box for the whole intersection, or do each lone signal communicate and keep in sync with the others?

I think temporary construction signals (for when two lanes narrow into one lane, which cars alternatively have to use) use radio to communicate, right? Or maybe just accurate clocks?

ash_wipe15 karma

One big relay box. (which are electronic relays nowadays)

Yeh, radio. You can see the antenna usually. Keep an eye out for it!

allwecaretopay10 karma

In some countries, the green light flashes before turning yellow. Do you think the US will ever adopt a system like this? Would it be hard to change? It seems like such a useful thing.

ash_wipe16 karma

I can't comment as I'm not from the US.

Reprogramming all the intersections would need to be done.

Charcolios10 karma

The software that decides what lights change and when. Is that located within the light itself and the lights just ping each other to receive updates? Or is that information centrally controlled offsite?

I've always thought it would be awesome to write the next best stoplight software to prevent rush hour and other traffics.

ash_wipe11 karma

Each site has a controller. Everything is stored on that basically. The lights are powered from that controller.

Controllers are all linked together creating a network that work together. In groups or independently etc.

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ash_wipe9 karma

Pm'd moderator.