Why Traffic Signals Are the Real Bottleneck of City Streets
How do Traffic Signals Work (2019)
Traffic signals are the unsung heroes of urban intersections, balancing safety, efficiency, and conflicting demands. This article explains how they work, from simple timed lights to actuated and adaptive systems that use sensors and machine learning to optimize flow. It also explores the limits of coordination and the promise of self-driving cars, revealing why congestion is so hard to solve.
In most cases, these intersections are the limit to the maximum throughput of the roadway. In other words, increasing the number of lanes or the speed limit won’t have any effect on the overall capacity of the road.
- programmertote
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.
- efavdb
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.
- stronglikedan
I highly recommend Traffic Light Doctor on YT, if you're into this sort of thing.
- Sohcahtoa82
> 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.
- WalterBright
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 :-)