Here's a surprising anecdote I've discovered living in South Africa wrt traffic lights.
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.
The two-adjacent signals causing extra traffic is something I encounter daily in Berkeley -- Ashby / Claremont & Ashby / Domingo. I probably blow an average of 5 minutes a day cause of that double intersection.
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.
Reminds me a bit of the MIT study that relates to spacing in densely packed roadways [1]. Whenever I apply the rule (look into my rear view mirror and decide how late to slow down), I feel like a good little molecule in an ideal gas.
One thing that I noticed about traffic lights in CA -- there will be like 4-5 cars who are driving on the major road and approaching the traffic light. Then a car arrived from the small road at the intersection. The traffic light will then turn red for cars on the major road, so all 4-5 cars will brake and stop at the light. The small car proceed to turn left into the major road. A few seconds later, the light for the major road turns green again and the 4-5 cars proceed.
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.
> Actuated signal control [...] inductive loop sensors embedded into the road surface
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.
There's a major arterial near my house. One day, the power went out in the area, and the traffic lights went dead. I discovered to my amusement that throughput on the arterial and its crossings increased dramatically, as drivers simply cooperated with each other.
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 :-)
I always thought that traffic signals should be trying to minimize squared anger, where anger increases linearly with time spent waiting at intersection.
I live in a small city in rural area (Nevada City/Nevada County, ca).
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.
>As sensors become more ubiquitous and computing power increases, traffic management may slowly but surely be relegated from civil engineers to software developers and data scientists. But, that also means that ASCT systems may be more vulnerable to security threats, a scary thought if they’re controlling the signals for an entire city.
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.
ASTC systems do not "dramatically reduce congestion." They result in a small (probably single digits) increase in traffic flow at best, which is a rounding error. God, I'm sick of this dude's flyover videos and him being treated like some genius.
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.
What I find most interesting about the computer side of traffic signal controllers is the interlock design needed to prevent a glitch or memory fault from activating conflicting phases at once, eg two intersecting approaches getting green simultaneously. That's about as safety-critical as it gets for a piece of everyday infrastructure. Would've liked to see more depth on that, though I imagine the specifics are controller/vendor-specific.
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.
How do traffic signals work? (2019)
(practical.engineering)86 points by at1as 18 hours ago | 63 comments
Comments
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.
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.
[1] https://www.csail.mit.edu/news/improving-traffic-tailgating-...
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.
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.
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 :-)
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.
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.
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.
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.