When Lightning Strikes a Traffic Light

[Homer J Simpson]

Video on troubleshooting and repair of traffic light control system after a lightning strike.

 

[Gyro]

It seems very cavalier the way he is swapping stuff on a live junction, triggering the green-conflict shutdown at least twice, etc. That would never be allowed on a UK junction fault find - there would be warning signage before and at the signal head poles, all signal heads obscured with thick orange oilskin covers etc. There are strict requirements for such things over here. We don't have the flashing amber junction fault indication (which I noticed he just turns off and ignores half way through), a failed UK junction (controller fault, Green conflict, or insufficient Reds) goes dark.

It's interesting to see country differences internally (I notice that it uses a Siemens controller though) and operationally, but his fault finding procedure looks to be dodgy as hell.

[Halcyon]

The green-conflict situation did seem rather odd.

Flashing amber is a thing in Australia. It's a failsafe mode where if the lights aren't operating due to some kind of fault (or if all power is lost and the lights are dark), vehicular traffic reverts to a condition as if the lights didn't exist at all. i.e.: Stop/Give Way (Yield for you Americans). Road Rules 101 if you will. There are also stop lines/signs at every traffic light in the country which enforce/remind people of those rules. It's one of the first things you learn as a learner driver.

I've done some work in the past with traffic light phasing and signalling, including some work on PLCs used those types of controllers, but I'm by no means an expert. The traffic lights here are based on a system called SCATS. This is an introduction (the COVID stance mentioned in the video is irrelevant, it can apply to holiday periods, public holidays etc...):



SCATS is interesting as it can operate in a variety of different modes: Isolated/Uncoordinated mode, Flexi-Link and Master-Link. TRAFF is the actual software that runs on the SCATS controllers, which can also run completely independently, due to comms failure etc... So a break in the fibre line isn't going to cause traffic chaos somewhere several hundred kilometres away.

[themadhippy]

Flashing amber

In the uk we only have flashing amber on traffic lights at a pedestrian crossing it means the you can go as long as the moving targets (pedestrians) are  clear of the crossing

[IanB]

Flashing amber is a thing in Australia. It's a failsafe mode where if the lights aren't operating due to some kind of fault (or if all power is lost and the lights are dark), vehicular traffic reverts to a condition as if the lights didn't exist at all. i.e.: Stop/Give Way (Yield for you Americans). Road Rules 101 if you will. There are also stop lines/signs at every traffic light in the country which enforce/remind people of those rules. It's one of the first things you learn as a learner driver.

In the USA the failsafe mode tends to be flashing red, which makes the light get treated as a stop sign (effectively turning the whole intersection into a 4-way stop arrangement). Interestingly, when this happens the overall traffic flow is about the same as when the lights are operating normally.

[tom66]

It's always struck me as odd that the UK doesn't have flashing amber/flashing red (Except flashing amber on ped crossings, but that's only for old PELICON/'Pelican' crossings.  Newer ones don't do the yellow flash, and pelicans are being slowly replaced.)

In fact, when traffic signals are out, the junction doesn't default to a STOP or anything like that, it just becomes a complete free-for-all.  No one has any defined priority, and you just have to make your way through traffic.  A four-way stop (or however many arms there are) would make more sense, but those aren't legal in the UK. 

Another issue is with roadworks and radio-controlled signals.  Typically, each signal is battery powered and the batteries have a runtime of 2-3 days in good conditions.  The signals all talk to each other on a private RF band.  All well and good, except when one signal goes off the network because the battery runs flat over the weekend for instance.  It turns off, and all the other signals on the network go red, because they can't be sure if the other phases are showing green or not.  A far better option would be to flash red - which could mean "STOP and proceed if clear" - but there's no provision in law for that.  I've seen a number fail in this manner and it completely backs traffic up for miles sometimes, because no one is sure if they should go and break the law, and the junctions rarely have space for more than one car at a time.  The last time it happened a police officer eventually attended and just rotated the signals round by 90 degrees and normality was more or less restored after a short delay.

[Gyro]

The green-conflict situation did seem rather odd.

Flashing amber is a thing in Australia. It's a failsafe mode where if the lights aren't operating due to some kind of fault (or if all power is lost and the lights are dark), vehicular traffic reverts to a condition as if the lights didn't exist at all. i.e.: Stop/Give Way (Yield for you Americans). Road Rules 101 if you will. There are also stop lines/signs at every traffic light in the country which enforce/remind people of those rules. It's one of the first things you learn as a learner driver.

I've done some work in the past with traffic light phasing and signalling, including some work on PLCs used those types of controllers, but I'm by no means an expert. The traffic lights here are based on a system called SCATS. This is an introduction (the COVID stance mentioned in the video is irrelevant, it can apply to holiday periods, public holidays etc...):

...

SCATS is interesting as it can operate in a variety of different modes: Isolated/Uncoordinated mode, Flexi-Link and Master-Link. TRAFF is the actual software that runs on the SCATS controllers, which can also run completely independently, due to comms failure etc... So a break in the fibre line isn't going to cause traffic chaos somewhere several hundred kilometres away.

Interesting video.

Yes, I'm rather out of date too, it was back in the early '90s that I was designing hardware for controllers. Interestingly (maybe), the market wouldn't bear the cost of PLCs in controllers, ours had proprietary single board CPUs (ours was embedded 80186 with 8051 watchdog (+H/W Green conflict backup), The lamp drivers were chassis mounted too. The only things rack mounted were the loop detectors and, primitive by today's standards, comms gear. We use mainly loops, supplemented Microwave and active IR (vehicle) and PIR (Peds). Ours also didn't have any in-cabinet UI (No PLC again), just using a plug-in handheld terminal. Setup of junctions and phases was done using a an in-house configurator system with the relevant local authority traffic engineer in attendance, with minor changes done on request, after reality was compared with expectation.

We had coordinated controllers too, both autonomous and centrally controlled, one of ours was SCOOT, aimed at moving 'packets' of traffic through a chain of intersections without interruption - as long as they obeyed the speed limit (otherwise they would be 'penalised' by hitting Reds). Predictably central London (TCSU) were the drivers for new systems of control and remote monitoring.

I'm not sure if we are unique in having part-time signals. These are normally on roundabouts ('rotaries' in the US), mostly on highway junctions. These operate both on time and on loop information, using a set of various priority loops, down to the furthest 'demand' loops, on the off-ramps, to bring the signals into operation early to prevent traffic from spilling back onto the highway, [Edit: again 'rules of the road apply, stop lines, priorities etc.]

I left the traffic industry just as the very early LED signal heads came in, after usable true Green LEDs became available.

[janoc]

It's always struck me as odd that the UK doesn't have flashing amber/flashing red (Except flashing amber on ped crossings, but that's only for old PELICON/'Pelican' crossings.  Newer ones don't do the yellow flash, and pelicans are being slowly replaced.)

In fact, when traffic signals are out, the junction doesn't default to a STOP or anything like that, it just becomes a complete free-for-all.  No one has any defined priority, and you just have to make your way through traffic.  A four-way stop (or however many arms there are) would make more sense, but those aren't legal in the UK. 

That's rather nuts. When I was in the UK I have noticed the literal jungle of traffic signs everywhere (compared to the continent), why not have some indicating main/side road on junctions for when the traffic lights aren't working? And if there is nothing else, you don't have a "left-hand" rule? I.e. traffic arriving to an otherwise unmarked junction has to yield to the traffic coming from the driver's left side?


That's how it works pretty much everywhere on the continent in Europe. The flashing orange light on the traffic lights when they are off (e.g. during low traffic times like during the night) is not mandatory, it has the same meaning as if the lights were completely off or not installed at all - rules revert to signage and right hand rule if the priority isn't indicated otherwise. I don't believe I have ever seen a junction with traffic lights - but with no signs indicating which road is the major and which one is the minor one. That people often don't know how to negotiate a large junction when the lights are out is another matter but the rules are fairly clear about who has priority.

[themadhippy]

SCATS

sounds like a pile of poo

you don't have a "left-hand" rule? I.e. traffic arriving to an otherwise unmarked junction has to yield to the traffic coming from the driver's left side?

that would be fun in the uk

[Gyro]

I remember discovering 'Priorité à droite' for the first time in France. New underwear time - it seemed very peculiar.

It is difficult to think of a junction in the UK that doesn't have priority line markings, even if they are a bit faded. Minor onto more major roads have Give Way signs as well as markings. We do have give way to the right system, but only on roundabouts, mini or otherwise. We have no equivalent to the US 4-way stop, one road is always major relative to the other.

The one exception that I can think of (until corrected!) is the failed traffic signals one. Where everyone proceeds with extreme caution and hopefully good manners.

[IanB]

For the most part, mini-roundabouts in the UK fill the same role as 4-way stops in the USA.

I far prefer mini-roundabouts. I think the ubiquity of stop signs in the USA is a major reason everyone there drives automatics.

On the other hand, there is an interesting game you can play in the UK called "hunt the stop sign". It's like a treasure hunt. You could probably spend a day looking before you could find one. And maybe not even then.

[Gyro]

Stop signs are relatively rare in the UK. They are reserved for junctions that have particularly poor visibility and there is a requirement to bring the vehicle to a complete standstill. It is also distinguished by a solid stop line and the word STOP on the road...



The one you should see everywhere is the Give Way sign. This is similar, but it is not strictly necessary to bring the vehicle to a complete standstill (although most people do, and if there is any relevant traffic, you need to anyway). The 'Give Way' line is a heavy dotted double line and an inverted triangle on the road. You should be able to see these everywhere, if you meet the sight requirements to legally drive ...





The Give Way sign can also appear just before the junction with a label indicating that you should give way in so many yards.

[IanB]

Stop signs are relatively rare in the UK.

But you found one! You win the prize! 

[Gyro]

Stop signs are relatively rare in the UK.

But you found one! You win the prize! 

(Hushed sound of 'Ohhh that's what that triangular sign meant').

Ha, yes I got that one -  I couldn't find a half decent photo of a Give Way sign and accompanying road markings on the web though! 


Edit: Come to think, there are a couple of Stop signs within a couple of minutes of my home if you want to start a collection.

[tom66]

It's always struck me as odd that the UK doesn't have flashing amber/flashing red (Except flashing amber on ped crossings, but that's only for old PELICON/'Pelican' crossings.  Newer ones don't do the yellow flash, and pelicans are being slowly replaced.)

In fact, when traffic signals are out, the junction doesn't default to a STOP or anything like that, it just becomes a complete free-for-all.  No one has any defined priority, and you just have to make your way through traffic.  A four-way stop (or however many arms there are) would make more sense, but those aren't legal in the UK. 

That's rather nuts. When I was in the UK I have noticed the literal jungle of traffic signs everywhere (compared to the continent), why not have some indicating main/side road on junctions for when the traffic lights aren't working? And if there is nothing else, you don't have a "left-hand" rule? I.e. traffic arriving to an otherwise unmarked junction has to yield to the traffic coming from the driver's left side?

That's how it works pretty much everywhere on the continent in Europe. The flashing orange light on the traffic lights when they are of (e.g. during low traffic times like during the night) is not mandatory, it has the same meaning as if the lights were completely off or not installed at all - rules revert to signage and right hand rule if the priority isn't indicated otherwise. I don't believe I have ever seen a junction with traffic lights - but with no signs indicating which road is the major and which one is the minor one. That people often don't know how to negotiate a large junction when the lights are out is another matter but the rules are fairly clear about who has priority.

Yes, it's just one quirk about the UK's traffic signals.  The other one being the red-amber get ready indication which probably made sense back in the days of heavy clutches and handbrakes, but even in my manual Peugeot I could get moving within a second, so it does seem a bit obsolete nowadays.  (Red-amber doesn't permit any movement;  drivers are still only allowed to proceed on green.)  In a future with EVs and only automatic ICEs, it'll look even more out of place.

The only case I know of traffic signals with signage indicating some kind of priority are at complex junctions where an outage would be really, really bad, like this one:
https://maps.app.goo.gl/chNxh3CpRz5Bihz29

But, they are very rare indeed.  I've only seen a couple in my ten years of driving.

[IanB]

The other one being the red-amber get ready indication which probably made sense back in the days of heavy clutches and handbrakes, but even in my manual Peugeot I could get moving within a second, so it does seem a bit obsolete nowadays.

Actually, when you drive in the USA you can appreciate why red-amber exists. When the light goes instantly from red to green (as it does over here), you can blink and you miss it, which leaves you thinking "Oh, it's green, when did that happen?"  Basically, your eye responds much better to movement than to a fixed and unchanging scene. So if the timing of your blink happens to coincide with the change from red to green, your brain won't instantly register that anything has changed, because the blink blocks out the movement of the light.

[Someone]

I'm not sure if we are unique in having part-time signals. These are normally on roundabouts ('rotaries' in the US), mostly on highway junctions. These operate both on time and on loop information, using a set of various priority loops, down to the furthest 'demand' loops, on the off-ramps, to bring the signals into operation early to prevent traffic from spilling back onto the highway, [Edit: again 'rules of the road apply, stop lines, priorities etc.]

Unusual but they do exist in Australia (historical roundabout example below), more commonly now as onramp metering:
https://en.wikipedia.org/wiki/Ramp_meter
(another rant for another day)

[thm_w]

Anyone know what the new cost of this equipment is? I can only find used Siemens M60 for a few hundred, but I'm sure new its thousands.

You'd think poles get hit often enough that not that much would fail. Or maybe they do but just not as hard as this one hit.

[Halcyon]

I'm not sure if we are unique in having part-time signals. These are normally on roundabouts ('rotaries' in the US), mostly on highway junctions. These operate both on time and on loop information, using a set of various priority loops, down to the furthest 'demand' loops, on the off-ramps, to bring the signals into operation early to prevent traffic from spilling back onto the highway, [Edit: again 'rules of the road apply, stop lines, priorities etc.]

Unusual but they do exist in Australia (historical roundabout example below), more commonly now as onramp metering:
https://en.wikipedia.org/wiki/Ramp_meter
(another rant for another day)

Common on some Sydney motorways, such as the M4. I've driven on this road a few times when ramp metering is in force and it actually does make a huge difference to traffic flow and allows vehicles to merge at close to the prevailing speed limit. I can see why some people hate it on face value, but it makes everything flow at a more consistent rate compared to just letting every idiot driver do their own thing.

Back onto the subject of traffic lights, when I was in the cops, I remember one intersection where the lights had failed and a technician came out. He opened the roadside cabinet and under all the equipment there were normal 100w (?) incandescent light bulbs dangling in series with each of the traffic light groups. I can't recall which colour it was actually connected to, but I suspect that when the lights themselves were swapped over to LEDs, the current draw was too low so was triggering a "lamp out" fault. That's only my guess as to why they did this, perhaps someone who worked in this gear can confirm? I may have a photo somewhere, if I can find it, I'll post it.

[Someone]

I'm not sure if we are unique in having part-time signals. These are normally on roundabouts ('rotaries' in the US), mostly on highway junctions. These operate both on time and on loop information, using a set of various priority loops, down to the furthest 'demand' loops, on the off-ramps, to bring the signals into operation early to prevent traffic from spilling back onto the highway, [Edit: again 'rules of the road apply, stop lines, priorities etc.]

Unusual but they do exist in Australia (historical roundabout example below), more commonly now as onramp metering:
https://en.wikipedia.org/wiki/Ramp_meter
(another rant for another day)

Common on some Sydney motorways, such as the M4. I've driven on this road a few times when ramp metering is in force and it actually does make a huge difference to traffic flow and allows vehicles to merge at close to the prevailing speed limit. I can see why some people hate it on face value, but it makes everything flow at a more consistent rate compared to just letting every idiot driver do their own thing.

Done well, ramp meters can pick the gaps in arriving traffic platoons/bunches and really improve the merging experience along with better safety and higher throughput. The ones I've observed in Australia were however just dumb timers to rate limit the onramp, which does have throughput improvements downstream, but if people will accept that then it should be a no brainer to go for the full benefits.

[Halcyon]

I'm not sure if we are unique in having part-time signals. These are normally on roundabouts ('rotaries' in the US), mostly on highway junctions. These operate both on time and on loop information, using a set of various priority loops, down to the furthest 'demand' loops, on the off-ramps, to bring the signals into operation early to prevent traffic from spilling back onto the highway, [Edit: again 'rules of the road apply, stop lines, priorities etc.]

Unusual but they do exist in Australia (historical roundabout example below), more commonly now as onramp metering:
https://en.wikipedia.org/wiki/Ramp_meter
(another rant for another day)

Common on some Sydney motorways, such as the M4. I've driven on this road a few times when ramp metering is in force and it actually does make a huge difference to traffic flow and allows vehicles to merge at close to the prevailing speed limit. I can see why some people hate it on face value, but it makes everything flow at a more consistent rate compared to just letting every idiot driver do their own thing.

Done well, ramp meters can pick the gaps in arriving traffic platoons/bunches and really improve the merging experience along with better safety and higher throughput. The ones I've observed in Australia were however just dumb timers to rate limit the onramp, which does have throughput improvements downstream, but if people will accept that then it should be a no brainer to go for the full benefits.

I'd imagine the ones on the M4 at least would be getting data from the roads/intersections feeding the motorways, to avoid those blocking up entirely. I can't be for certain but it makes sense to do it this way solely because there is no specific time of day when ramp metering is enabled/disabled, it's entirely dependant on traffic volume and flow.

[Someone]

I'm not sure if we are unique in having part-time signals. These are normally on roundabouts ('rotaries' in the US), mostly on highway junctions. These operate both on time and on loop information, using a set of various priority loops, down to the furthest 'demand' loops, on the off-ramps, to bring the signals into operation early to prevent traffic from spilling back onto the highway, [Edit: again 'rules of the road apply, stop lines, priorities etc.]

Unusual but they do exist in Australia (historical roundabout example below), more commonly now as onramp metering:
https://en.wikipedia.org/wiki/Ramp_meter
(another rant for another day)

Common on some Sydney motorways, such as the M4. I've driven on this road a few times when ramp metering is in force and it actually does make a huge difference to traffic flow and allows vehicles to merge at close to the prevailing speed limit. I can see why some people hate it on face value, but it makes everything flow at a more consistent rate compared to just letting every idiot driver do their own thing.

Done well, ramp meters can pick the gaps in arriving traffic platoons/bunches and really improve the merging experience along with better safety and higher throughput. The ones I've observed in Australia were however just dumb timers to rate limit the onramp, which does have throughput improvements downstream, but if people will accept that then it should be a no brainer to go for the full benefits.

I'd imagine the ones on the M4 at least would be getting data from the roads/intersections feeding the motorways, to avoid those blocking up entirely. I can't be for certain but it makes sense to do it this way solely because there is no specific time of day when ramp metering is enabled/disabled, it's entirely dependant on traffic volume and flow.

That would be the complete opposite of how a ramp meter should work, they should congest and fully block the feeding roads to maintain free flow on the highway.

[BrianHG]

It seems very cavalier the way he is swapping stuff on a live junction, triggering the green-conflict shutdown at least twice, etc. That would never be allowed on a UK junction fault find - there would be warning signage before and at the signal head poles, all signal heads obscured with thick orange oilskin covers etc.

Yes, this thought was running through my head too as I watched the video....
I know here in Montreal, there are fold out emergency stop signs on most traffic light poles at any major intersection.

And you would think there would be a mechanical breaker for all lights in the control box allowing for such tests to be done with all the lights off.

I guess everything is built to a minimal cost.

[Halcyon]

I'm not sure if we are unique in having part-time signals. These are normally on roundabouts ('rotaries' in the US), mostly on highway junctions. These operate both on time and on loop information, using a set of various priority loops, down to the furthest 'demand' loops, on the off-ramps, to bring the signals into operation early to prevent traffic from spilling back onto the highway, [Edit: again 'rules of the road apply, stop lines, priorities etc.]

Unusual but they do exist in Australia (historical roundabout example below), more commonly now as onramp metering:
https://en.wikipedia.org/wiki/Ramp_meter
(another rant for another day)

Common on some Sydney motorways, such as the M4. I've driven on this road a few times when ramp metering is in force and it actually does make a huge difference to traffic flow and allows vehicles to merge at close to the prevailing speed limit. I can see why some people hate it on face value, but it makes everything flow at a more consistent rate compared to just letting every idiot driver do their own thing.

Done well, ramp meters can pick the gaps in arriving traffic platoons/bunches and really improve the merging experience along with better safety and higher throughput. The ones I've observed in Australia were however just dumb timers to rate limit the onramp, which does have throughput improvements downstream, but if people will accept that then it should be a no brainer to go for the full benefits.

I'd imagine the ones on the M4 at least would be getting data from the roads/intersections feeding the motorways, to avoid those blocking up entirely. I can't be for certain but it makes sense to do it this way solely because there is no specific time of day when ramp metering is enabled/disabled, it's entirely dependant on traffic volume and flow.

That would be the complete opposite of how a ramp meter should work, they should congest and fully block the feeding roads to maintain free flow on the highway.

I don't agree. Ramp metering is designed to regulate traffic flow, not just onto the on-coming highway but from those lower-speed roads feeding it. Without it, some roads would be chaos.

Prospect Highway southbound is a prime example of that. Without proper metering and traffic light phasing, significant traffic builds in the right turn lanes (onto the M4). There is plenty of "buffer" on the on-ramp itself, but there isn't a lot of room for vehicles along Prospect Highway. Roper Road is another example where there is only one turn lane onto the on-ramp (westbound) where you might only fit 10-15 cars at a time. If that lane fills up and you have more vehicles waiting to enter that lane, all southbound lanes along Roper Road are completely blocked and no one can go anywhere, until those entering the highway are cleared.

If the roads feeding the on-ramps/highways are all blocked up, then there would be little point to having metering on the on-ramps themselves. This is also why you see sensors and induction loops on the roadway not just at a set of traffic lights, but well before it. This allows the system to essentially know how many vehicles over any given time are occupying certain parts of the road network.

Ramp metering impacts several roads, not just one. Generally speaking, slower more consistent traffic during peak times is better than letting people just do their own thing. You always get some numbnuts who have no idea how to merge, leave it until the last moment, realise they've run out of lane and come to a complete stop. When you have thousands of vehicles per hour entering intersections during some parts of the day, this has a huge impact on traffic behind them. This is why phasing needs to be constantly monitored and adjusted. There is also little sense in having a green traffic signal that only leads onto a congested road. You might as well allow the traffic that can flow, to do so.

As police, we would occasionally manually alter phasing on primary and surrounding roads to account for things like lane closures, crashes, road blockages etc... Things that SCATS isn't really designed to (or can't) detect.

Not specific to ramp metering, but there's an interesting document by the NSW Government published in October 2023 about the M4 East Road Network performance.

[Someone]

That would be the complete opposite of how a ramp meter should work, they should congest and fully block the feeding roads to maintain free flow on the highway.

I don't agree. Ramp metering is designed to regulate traffic flow, not just onto the on-coming highway but from those lower-speed roads feeding it. Without it, some roads would be chaos.

Lol, and this is why I say ramp metering is dumb and stupid as it is used in Australia.

Over all users time is saved if the highway can be kept free flowing. But to do that you need to keep demand constrained by congesting somewhere (mentioned in the wikipedia article). As soon as "cant let the feeders get congested" is a requirement then the entire point of increasing overall network throughput is lost.

Classic car brain... "just one more lane and the congestion will be gone, honestly this time for sure". Adding more flow into congested systems makes it worse (past saturation) and the only controls available are tolling or adding congestion to reduce demand.