How do traffic signals work? (2019)
practical.engineering
practical.engineering
I have seen this so often that I sometimes wonder if it'd be more fuel efficient to let the 4-5 cars on the major roadway proceed (the traffic lights have cameras anyway) while letting the one car from the small street wait slightly longer or wait for a fixed amount of time.
I wonder if there's any traffic engineer on HN from California and if so, I'd love to hear an explanation/rationale behind the current approach.
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.
Duh. Cultural differences in English are amusing.
With such a setup, the traffic lights controller can't actually see that there's only 4-5 cars approaching on the main road – either they're far away enough that they don't even register on the detector for the main road yet (in which case a vehicle waiting on the crossroad will naturally trigger the phase change without delay), or even if they're already near enough to the intersection to trigger the detector, the traffic lights controller can't actually know that it's "only" 4-5 vehicles and its strategy might hence be that if the main road has had more than x seconds of uninterrupted green time, a demand from the side street will be granted immediately, regardless of traffic conditions.
With a state-of-the-art system it'd be different – with more and better sensors the traffic lights controller could actually track individual vehicles (or platoons of vehicles) as they approach the intersection and try making the kinds of trade-off you mention – but somebody needs to pay for that kind of upgrade. (And I suspect we haven't necessarily reached the point where the price premium for that kind of advanced traffic lights controlled compared to classic traffic actuation is negligible enough for it to be the default at least for all new intersections and equipment renewals, either…)
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 and that's a recipe for a growing line, which causes a traffic jam.
The Old County lights at least are staggered so they are almost always green at the same time. The Central Expressway ones, not so much - there've been times I've been stuck for 20 minutes trying to cross the 100 feet from Moffett to Castro in Mountain View, because the light timing never lined up with the train signals.
They've conveniently solved this by closing the Moffett->Castro side of that intersection entirely. You can still go Castro->Moffett, but they're planning on closing that to eliminate the grade crossing entirely.
Is there still pedestrian access from the northeast side of the Caltrain? I used to live in the Moffett/Middlefield area and being able to walk to downtown was a big draw for that.
Yeah, they're planning to put a ramp from the Shoreline bridge over the tracks down to Evelyn to spread out the traffic, but right now people take Villa.
> Is there still pedestrian access from the northeast side of the Caltrain?
Yeah, all the crosswalks are still operational. The long term plan is to put a bike/ped tunnel under Central and to completely close off the Castro/Caltrain side.
The second reason: trying to incentivize people onto trains, buses, bikes, foot, etc.
Cambridge, MA does this. The light are purposefully timed to not let you travel through multiple greens on the tertiary roads in particular, less so secondary, etc.
Cambridge also gives a substantial amount of time to pedestrians as part of every cycle- no "beg button", because the city does not consider someone on foot to be less deserving of crossing time - specifically to not have the car "be king". By making it relatively fast to get around on foot, they reduce the incentive to drive.
They also heavily structure the residential neighborhood streets with one-way directions designed to make it very difficult to use the streets people live on as cut-throughs when the main roads jam up because that leads to noise, pollution, and danger to people on foot and bike, particularly because the kind of person to use a neighborhood as a cut-through is likely also driving quickly and aggressively.
Ideally, 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 intersection lane will have to have less then ideal throughput because you don't want to overwhelm the next section, as this could be even worse for the system overall.
If you have some engineering background you can also think about this as impedance matching. If you have a 4 lane road narrowing to 2 lanes you have a sudden impedance increase, leading to flow issues. If you put an intersection inbetween you now have a way to manage that impedance mismatch with signaling times, but you have to both look at the impedance before and after the intersection.
Who claimed it was tho? We were talking about throughput. If all your intersection knows is time, it can't decide based on traffic. Which is why inductive loops, camera based systems and so on exist, but (1) they are not deployed everywhere and (2) if they are deployed there may still be a time based limit for the case of defective sensors.
Our traffic systems are already pretty well optimized where we are willing to spend money on them. Note however that if you 100% optimize any system for perfect throughput, it has zero slack to deal with unexpected fluctuations in traffic patterns. That is as true for traffic as it is for networks. Throughput is important, but so is comfort, resilience, predictability and cost. All different metrics you have to keep in mind¹ and all of them can suffer when you over-optimize for one.
The view from inside the traffic that we are part of may make that seem inefficient, but seen system wide the main problems usually are not traffic lights, but harder to change structural issues.
¹: or rather you don't have to keep them in mind, since there are people who specialize on just that and are pretty good at it
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 operable.
I've timed this a few times.
Of course, it helps that infrastructure adjustments over the decades ensures that many people that don't need to be in cars, aren't in cars.
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 situation is different? Not sure.
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 is however one intersection (Stirling and Balmoral Ave) on the way to my kid's school on a non-major road which definitely works better when its traffic light is out, and this is at any time of day in my experience.
Some traffic lights definitely should rather be mini traffic circles, e.g. on Peter Place by the one office park and had the exact same frustration for the one on Grosvenor Road near to Bryanpark.
As the Johannesburg Road Agency (like all City of Johannesburg Municipal companies) is so incredibly incompetent, our traffic lights are incredibly unreliable, I shudder to think how much it sucks out of our economic output all the people sitting in traffic longer than is needed simply due to an out of order traffic light.
And the local police also don't help with regular roadblocks while at the same time being terrible at law enforcement (yes, you'd think roadblocks are a sign of law enforcement, in practice though they're more for soliciting bribes while screwing up traffic).
[1] https://www.csail.mit.edu/news/improving-traffic-tailgating-...
It 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 in a row on a major roadway. If one signal gives a green but the next one doesn’t, cars can back up
This is a major problem at an intersection near me that I used to have to drive through frequently. Two intersections, only about 500 feet between them, and MOST traffic during rush out coming from just one of the directions, yet the lights were often exactly out of sync. The first light would be red while the next one was green. Then the first would go green while the other went red, and traffic would only move about 500 feet then stop for 2 minutes. The backup would last well over a mile.
> The obvious next step in efficiency is coordination of most or all the signals within a traffic network. This is the job of adaptive signal control technologies, or ASCT. [..] These types of systems can dramatically reduce congestion
My cynical ass would not be surprised at all if oil/gas companies were lobbying against ASCT because idling cars are buying more fuel. Wouldn't be too hard to convince people that an ASCT would cost too much to implement while not producing significant traffic benefits.
In San Francisco (as well as other cities I've lived in) they put police in the middle of intersections during rush hour to guide traffic (i.e., stop cars before they drive into an intersection that they'll get stuck in the middle of). Seems woefully inefficient
Imagine how much gas would be saved if the city spent a little effort on the traffic lights.
The bike traffic remains 1% of the car traffic, and 0% when it's cold/rainy.
This sentence alone tells me that it's not a bike lane. A painted line is not a bike lane.
Walter I'm seriously fighting the urge to write a wall of text in response. Why the knee-jerk reaction and the hyperboles? It's not interesting conversation...
It's not really saving gas if your alternative doesn't achieve even remotely the same thing.
In the USA, a significant amount of tax-funded infrastructure is actually built out by contractors, which means there's profit going to a corporation, and there's often a cynical idea that the contracts go out to whoever will send a kickback in the form of a campaign contribution to whoever makes the decision on which contractor to use.
That means all the bribery has to be firmly under the table instead, rather than legalized.
I don't think you need a vast conspiracy to point out that connecting traffic signals costs more (fancier signal equipment, running cable, maintaining the cables, etc) and the benefits may be subtle, hard to predict, and maybe even hard to measure.
Additionally, you have a lot of people that would prefer to slow traffic. Depending on local culture, investing in traffic control can be difficult (although upgrading traffic signal systems may be better able to accomodate priority for transit and bicycle detection)
This is a bizarre conspiracy theory that you just invented. Stop making up things to get angry about.
And the roads themselves are good in NL (huge contrast when I moved to Montreal, where we'd have a metal mesh underneath bridges to catch falling concrete)
In the US, we have many more lights due to the road layout (grid). There are more than 3200 signaled intersections in my county in California alone despite being 1/14th of the land area of the NL.
The robot also estimates that there are 300,000 signaled intersections in the US. We honestly don't know how many there are because they are all operated by different agencies.
For US arterial roads iVRI has limited benefit. So the focus in the US has been on V2X development which focuses more on crash avoidance and has much higher installation costs due to latency requirements. In the end there's a lot of low hanging fruit here in the US if signals could just be retimed.
https://transportationops.org/publications/2012-national-tra...
NL has one procurement entity. In the US there is no single coordinating entity to standardize traffic lights. Each state standardizes its own lights and then each municipality handles its own procurement.
Lowering the number of traffic lights in and of itself is a good idea. Overpasses and (turbo)roundabouts, clever right of way, better road hierarchy; there are lots of alternatives that may provide better throughput and more safety for a given intersection.
The Netherlands as a country with 18M inhabitants is best compared to a big city with the surrounding metro area.
There's an annoying one near me at the bottom of a hill where anyone approaching from the sidestreet will ruin the descent on bicycle, even if they were turning right (from a dedicated turn lane) and didn't need the light to change for them at all. Another is a former police station now used as a park facility. Cars entering the parking lot only need to barely brush over the sensor and the light changes instantly for no reason because they never reprogrammed the light when the police left.
Have you really never seen what happens when something grows organically without global maintenance? Not saying that this is what happened in Germany, but "it takes work" sounds naive.
It doesn't take work to write a crappy codebase: all you have to do is add code without really caring about the overall architecture.
Or said differently, if you can fix the German traffic engineering, you should apply there because I'm sure they would be happy to have an easy and perfect solution.
From what I've seen about US traffic, there doesn't seem to be much to be envy of.
Most intersections are atrociously designed. Protected left turns everywhere, with their own lane and their own traffic light causing slowdowns and triggering also when nobody needs them.
Many pedestrian crossings could be moved 10m further from an intersection, allowing up to a couple of cars (wishing to turn right) to wait on the pedestrians without blocking all the people who were just going to proceed straight ahead. On the other hand, this problem would often not even occur if we hadn't wasted a lane for a protected left - there now would be a dedicated lane for those going right.
Right-on-red is legally permitted (there's a dedicated street sign for it) but almost never implemented. It's so frustrating.
An overwhelming majority of the traffic lights I encounter is completely non-adaptive. I wait an average of 40 seconds every morning at a specific one which is, in aggregate, causing entire human lifetimes of wasted time on a monthly if not weekly basis.
Zebra crossings are often outfitted with a pedestrian traffic light which is red (rather than off) by default. If the road is completely clear, you could just cross, but no, you must press, wait up to half a minute, and finally it turns to green for you. Now you also stopped a car or 2 which need to wait for your green to finish. If it defaulted to OFF, you could have crossed 20 seconds ago without interrupting anybody's life, and the traffic light would still still be available for elderly or disabled (or whomever wishes to press and wait, while not having it as a default).
...I could continue.
I feel like software engineers would write more secure software than civil engineers. It would be the opposite. I'm also not scared that much of someone controlling traffic lights. It's already possible using emergency lights.
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 optimizations drive the average driver f---kin nut. I don't think that cost should be ignored.
There are already cameras on top of all the lights. How hard can it be to hook them up to AI that is instructed to maximize throughput? The current system will block 3 directions while having a green light on the 4th which has no traffic.
I once complained to a friend about this, and he told me that the traffic engineers were experts and I was lying. The next day I was driving on that road, and got a red light. You could see on down the distance 3 greens. There was no other traffic, and no cross traffic. So I sent him a photo. He conceded :-)
You don't need AI for that, you just need the system configured properly. Lights near my parents' house have been running cycles adapted to actual traffic for at least two decades, probably three... although using inductive loops in pavement rather than cameras.
There are reasons you might want a short green every so often even with no detected traffic... it would help manage worst case situations for a stuck open detector, etc. But those wasted greens shouldn't be long enough to notice. You might also have situations where locally suboptimal behaviors improves performance or safety elsewhere and that may lead to a better overall system results. I'm not claiming this is the situation in your scenario, but it can explain some negative experiences.
Otoh, Milwaukee circa 2003 had all the lights on a schedule. Every intersection made the same changes every day. Super frustrating when I was consistently slow to get out of the house for a scheduled pickup and missed the light everytime.
Those are infrared sensors that pick up the heat from car engines. They've had to be modified to pick up electric cars.
> How hard can it be to hook them up to AI that is instructed to maximize throughput?
The equipment cabinet at the intersection has been engineered to prevent all sorts of fatal things that happened in the past. Such as one set of relays being stuck so that one direction always gets a green light while the other direction gets red/yellow/green. They're designed so that can never happen again (when "broken", you'll see blinking red in all directions).
The controllers need to be standardized so that you don't need experts to be able to diagnose & repair them. They also need to be certified. It is not possible to certify an expert system (or as they're now called "AI") because it is not possible to use formal methods to prove correctness.
> The current system will block 3 directions while having a green light on the 4th which has no traffic.
That specific intersection is set on a timer. For a single, 2-lane each way, non-complicated intersection, it can cost $100k for the lights, poles, sensors and equipment cabinet. Every wire is underground in pipes so that the expansion of concrete in weather won't stretch/break the wires.
> as drivers simply cooperated with each other.
That's something that most people learn in kindergarten: "share with others" and "take turns".
Where I live (fly-over country), when the power is out, traffic backs up significantly.
Disclaimer: I used to work for my state's combination DMV & highway department.
Do they pick up ICE motorcycles ? Electric motorcycles ?
Heck, an AI isn't even needed. A simple cost function could be easily developed, where each car accrues priority as it waits, and the lights go green on the ones with the highest priority.
Also red left turn signals should not be a thing. If there's a gap in the oncoming traffic you should be allowed to turn left across it as long as the straight-ahead traffic has a green light.
Where I live (in the PNW) they mostly are not a thing any more. The only ones remaining are there because the combination of sight line and traffic speed makes them too dangerous to turn into flashing yellow. Not sure if that has spread to other regions, but I would guess it is heading towards being universal pretty quickly.
The only possible second-or-third order effect of red cross-traffic arrows is to decrease the downstream traffic on the cross road. Mostly, the places where I encounter them have a low traffic cross road. IMHO, in those situations, red cross-traffic arrows are simply a failed "traffic engineering fad".
(Edited to fix a typo and generalize "red left-arrow" to "red cross-traffic arrow" to account for left-side-of-the-road driving systems.)
Numerous cities don't bother with ASTC systems because they a)don't do very much yet cost a lot of money in equipment and staffing, and b)just further incentivize low occupant vehicle traffic, which is just about the lowest priority in most cities these days because that in turn creates more traffic. If there was one overarching tenet to traffic engineering and urban transportation planning, it's that you cannot solve traffic by building more roads. It doesn't work. It has never worked.
Traffic is something 99% of the population thinks they understand because they sit in it, see it, etc. and probably 95% don't have the foggiest idea how it actually works, and merely have a severely uneducated and highly biased personal opinion. And boy howdy, do they hate bike lanes and bus lanes.
Adding (or removing) a lane does not even remotely have a direct correlation to maximum traffic flow. If you have a three lane road and you make one lane a bus or bike or bus-and-bike lane, you do not lose 1/3rd the traffic flow capacity. It's dramatically less than that.
By making bus transit faster (where each bus holds ~60 people...) and biking safer, not only do you actually reduce traffic, but you also provide emergency vehicles with a faster route through traffic.
In Paris more people travel by bicycle than car. That was a direct result of Paris heavily restricting motor vehicle traffic through the center.
Every person on a bicycle is one less person causing noise, pollution, congestion, wear to the road, danger to others, and using (at least) one less parking space (because you don't just need a place to put the car where it is kept, you also need space for where it is going.)
Simcity designers initially based Simcity off actual real-world stats and planning figures. They had to throw out the figures for parking and road capacity because otherwise the game would have looked like nothing but roads and parking lots.
Of course it does. What an obviously stupid thing to say. You think a 4 lane motorway doesn't allow more traffic flow than a single lane country road?
I think that interlock design also goes some way toward mitigating the security risks ASCT systems would otherwise pose, as mentioned in the article. On any decent controller, even the control room shouldn't be able to deliberately trigger conflicting phases.