How Do Traffic Light Sensors Work? | Did the lights fail?

In this video, we look at a situation that developed when my viewer was leaving a car park behind a police vehicle. Were these lights broken, or were there things that the motorists could've done to help the lights change?

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[deetee]

The policeman was very good to not just jump the lights and to attempt to sort the issue. Then, by getting community support officers involved and moving his car too.
Props to that officer for not being selfish! ❤

[steffenscheibler]

I worked for a company supplying road-telemetry and traffic management systems systems (global supplier, also to the UK). There are many different types of sensors but the UK predominantly uses motion detectors on small junctions such as this one because they are very cost-effective and work (mostly) rather well for those types of junctions. Larger or more complex junctions typically have better systems which will trigger on more than just motion and often also combine magnetic loops in the tarmac to detect if a vehicle is standing at the line (which is why you always should drive up to the line, not stop ~5 meters before it!).

[SGz_Eliminated]

I think Ashley was spot on, the traffic just doesn't move after its cycled back to red. The chances of the sensors breaking at that exact moment is extremely slim.

[CraigNiel]

I've been in this exact situation (without the Police though) where the red light just stayed on and would not change. I tried reversing and driving forward but nothing happened. After about 5 minutes I just had to go through the light as it seemed broken. I just treated it like any other junction and gave way to any traffic as you normally would. Luckily it was quite quiet and I just took it really slow and all was well.

It's so ingrained in us not to go through a red light (and rightly so) that it really does feel so strange to do it, even under such necessary circumstances.

[craigstevenson]

I'm just dropping a wee message to say thank you. I just passed my test, first time one minor with my dad as my only instructor. Your videos helped me in many situations!!!

[PedroConejo]

At least the Highway Code (checks notes) allows you to make a judgement call if you have good reason to believe the lights have stopped working: "If the traffic lights are not working, treat the situation as you would an unmarked junction and proceed with great care." - Rule 176.
Given the dashcam evidence, if there was no possibility of trying to reset them by reversing, I'd follow the advice given. Another reason to have a dashcam.
It is a good reason for auto-cycling the lights every few minutes, but that's for the engineers to implement. I don't listen to the radio these days, but I used to hear 'stuck on red' quite a lot in the traffic updates on my way to work.

[pauleff]

AS A CYCLIST - I *hate* jumping red lights.... But if they don't know I'm there because I'm not big enough / heavy enough / wide enough to "trip" the sensors, and it's 1 AM or so and there is absolutely no-one about WHAT DO YOU EXPECT ME TO DO? Sit there and freeze to death? I always, always stop for red lights but I won't wait longer than 20 seconds where and when the roads really are deserted. I listen, listen, listen and then set off again - usually cussing whilst doing so into the video camera about the traffic lights needing to work for me too!!!! I HATE jumping the lights and would just prefer that they work!

[davidhaslam]

Hands up if you would of simply reversed a few feet and moved forward to see if that worked. 🙋‍♂️

[dgs]

Backing away and approaching again would have fixed this. Also on the ground loop system with a motorbike stopping and starting again gives the system a bigger jolt

[antm]

This happened a few months ago with temporary traffic lights for construction work, the lights were situated at a blind bend, I had a copper behind me along with around 20 cars. After 8 minutes on red and no change or other traffic oncoming, I started to creep forward and went straight through with all the cars following, long behold they were manually put onto red purposefully as when we got around the corner there was a construction HGV blocking the road and using its crane to drop off materials. So we all put our tails between our legs and reversed like proper idiots 😂

[learigg]

The lights in Ayrshire are mostly controlled by induction loops buried in the road. A common issue is cars turning right not pulling forward on green light. There is a loop in the road where you wait to turn right, if you're not on top of this then the filter light will never activate. I've sat through 4 or 5 light sequences when the driver in front refuses to pull forward. We could definitely use some general training on how lights work and also reinforce thats its ok to pull forward when right exit is busy, assuming we're not on box junction.

[DaveHines]

As a motorcyclist, I’ve often found traffic lights don’t pick up my motorbike, and have to wait several minutes til they cycle . I recently got stopped for about 5 minutes at temporary traffic lights which kept on letting cars thru periodically from the other direction, they eventually changed to green when a police motorcyclist approached the lights passing the stop here sign .

[Kromaatikse]

This junction has both motion and inductive sensors. You can see the grooves for the latter pass under where the camera car stops the first time (diagonal, third car back) and also, crucially, *between* where the police car stops (halfway across the stop line) and where the camera car stops. This latter set of grooves go straight across the road, and are most easily seen where they interrupt the double-yellow painted lines.
I think the lights changed for the first time, not because there was a gap in the traffic, but because it sensed at least three cars waiting (both the first-car and third-car sensors active). Then it was confused, not by a lack of movement per se, but because the remaining cars stopped in positions where the installed sensors couldn't detect them. If the police car had reversed back so that it covered the first-car sensor, this would have prompted the lights to insert this road into the normal cycle sequence; the third-car sensor allows for a priority system, and I think the motion sensor is mostly used to judge when to revert to red.

[clickrick]

My late father worked in traffic management until he retired some 25+ years ago.
There are many variants of traffic light control, with sensors visible above (as here) and buried in the road (as you said), with time of day also a factor, but most have a default "error" mode that they will fall back to if they fail to sense anything for a set period of time.
As with everything, that period can vary, with times of 15 minutes not being uncommon in a city. In other words, if it doesn't sense traffic approaching for 15 minutes, the controller will assume there's a fault with the sensors and go to standard timed intervals. Whether it then goes back to sensor-driven is another variable, but you might see a small red light visible on the equipment somewhere which will indicate that it's in this error state.

[johnlochness]

One thing I've noticed with temporary traffic lights in recent years is that if the road is quiet they will turn to red for both directions, then when a car approaches they can instantly turn green for that direction.

[grahambullman]

These lights are 400 yds away from my house, and this happens a lot with them. They don’t recognise me when I’m on my bike. Normally two lanes of traffic line up, depending which lane is required around the bend which I think helps to keep things flowing and ensure there is generally some movement. 🙂

[johnkeepin]

While this is an exit from a public car park, there are some places where the exit only works for buses fitted with traffic signal priority (TSP) kit, which is used to detect them and sets the route for them. Also used to give them priority over certain junctions, in which case the sequencing of routes varies, rather than a standard one.

[ShedTV]

I agree with the theory that the sensor isn't being triggered. I think it's pointing at the bit of road between the Police car and your viewer. The Police driver got caught out as the lights changed quite quickly, because they're probably timed in favour of the main road. He couldn't clear the junction, leaving him stranded across the line. The other car is too far back to catch the sensor. I'm pretty sure if the Police officer had reversed up to the camera car it would have triggered the lights.

[captafrica]

Oddly enough, I had exactly this situation a few months ago, in a major city at night trying to join from a two lane, one way side road to a main road.
I waited at the lights for around 6 minutes in the left lane, realised something wasn't right when private hires blew through the lights while the rest of us waited.

Once a gap in the traffic developed, I proceeded through the lights treating it as an unmarked junction (As Rule 176 in the Highway Code states). Bad luck for me that an undercover police car had joined the queue around 30s before this and pulled me over once I'd passed the junction.

3 points and a fine with a solicitor advising me not to fight the FPN as technically there is no official guidance on how long a light needs to be out before we can consider it broken, and losing the fight in court could result in a much larger fine and a charge for driving without due care and attention.

I tried explaining to the officer that pulled me over that the light clearly was not working and I had several minutes of evidence of this on my dashcam. He wasn't interested and "kindly" let me know that I don't have the authority to determine that a light is broken and that I should wait at the red light for as long as it takes.

[lyndsnevircs]

I was required to wait at these lights yesterday. Even when we were stationary, the lights changed to green within 2 minutes. I would say they were faulty, they changed red very quickly aswell from my past experience.