Interesting traffic light casework

A few weeks ago, a primary school-aged FOCUS reader got in touch with me to complain that a pedestrian crossing on a very busy main road took ages to let him cross the road. Indeed so long, that he had got a stopwatch to time it and said he had been waiting over 10 minutes to cross.

Clearly not right, so I passed on the details to the traffic signals team at the Council and asked them to look into it.

The junction in question – which used to be called Kamikaze Island ((roundabouts are called “islands” oop ‘ere in Nottingham)) until there were so many accidents they eventually, shortly before I came to the city, gave in to a Lib Dem campaign and replaced the roundabout with a pretty complicated traffic light scheme.

The traffic lights are controlled by a system called SCOOT ((Split Cycle Offset Optimisation Technique)) and part of that means there are a lot of traffic cameras in the area to help people help the computer control traffic remotely from Traffic Control Centre (TCC), currently located in Lawrence House in the city centre. So the staff there looked at the system, tried to make some changes, and asked my constituent for feedback about whether it had solved the problem. Many thanks, by the way, to the officer in question for writing an answer that clearly explained all this in a way someone of his age could easily understand. I’m probably not managing that in this blog post!

Unfortunately, the problem was not fixed by the changes made remotely, so we agreed on a site visit. Me, the signals engineer and the young constituent would all meet up for a cup of tea at 8am, then we would walk to school together and see the problem in action.

When we did, it was fairly clear what was going on.

For most of the junction, it’s possible to have green man phases when the traffic is stopped. Because the junction is so busy, it’s not possible to have a phase when all the traffic is stopped, so there are two exit roads out of the junction where the traffic only stops if pedestrians press a button. To make sure the traffic only stops when necessary, the junction waits for the button to be pressed, and before stopping the traffic, detects whether the pedestrian is still waiting, using visual sensors near the button. If the pedestrian moves away from the road – or manages to cross in a gap in the traffic before the green man comes – the crossing automatically cancels the request for the green man.

The signals engineer demonstrated this to all of us – me, my initial correspondent and his brother and family – by pressing the button, and making all of us stand away from the edge of the road. The “Wait” light came on when he pressed the button, but when he moved out of the range of the sensor, it just went off again, without calling the green man phase.

When we asked the young man to show us how he used the crossing, it was clear that this was what was going on. He’d been taught to press the button and then stand well clear of the road edge. By standing away, the crossing stopped detecting him and cancelled his request.

This is not an unreasonable thing for him to be doing. Traffic does come past the roadway very fast, and it’s very safety conscious to step back from the danger zone. And if the crossing is not detecting this one person, there are probably many others similarly affected.

So the engineer took the decision to disable the sensors. Now if you press the button, you get a green man every time, and the crossing does not take into account whether it can see someone standing there or not.

Since the sensor equipment is expensive, it will be moved and redeployed somewhere else where it can be more effective – and where hopefully it can be better tuned to detect people.

For me, a bit of a nerd on issues like this, one of the more exciting bits of the morning was when the engineer reprogrammed the junction. He did this by taking a big yellow keyboard out of his bag, unlocking the grey system box on the roadside, and plugging the keyboard into the computer there in the box. I’ve never seen inside one of those boxes and was keen to peek. The thing I remember most was the unshielded transformer in the corner of the box stepping mains voltage down to the low voltage needed to power the environmentally friendly LED lights and the computer equipment. Never open or hit one of these boxes with your car or you could get electrocuted!

(This post – or at least the idea you can blog about junctions – partly inspired by Helen Duffett’s post The Slash!)

(Also sorry for the slightly stuffy “constituent” and “traffic signals engineer” – because of the Power of the Internets and for privacy reasons, I try and avoid naming people who are not public figures)


3 comments on “Interesting traffic light casework

  1. Penny says:

    Sounds like a very ‘smart’ system, seems a shame not to find some other way to make it work. How about a yellow box painted on the pavement and an addition to the pedestrian sign to say ‘push button and wait in yellow box’?
    We had a small and efficient roundabout here replaced by traffic lights which fail to keep traffic flowing. Apparently there is a ‘smart’ system installed to control things better but this has not been switched on for reasons which remain unclear 😦

  2. Well-written and charming piece, Niles (hope that doesn’t sound patronising). As an advocate of traffic system reform, I wonder if the problem was amenable to a low-tech solution. It certainly is a big, and no doubt often busy junction, but what if the rules of the road were changed to reflect our social nature? What if we replaced priority with equality, so instead of rights-of-way based on status of road or direction of travel, we took it in turns based on time of arrival? Without standard priority and signal control, would survival instinct and social custom see to it that everyone approached carefully and filtered more or less in turn? There are some answers at FiT Roads, including Parts 1 and 2 of a video. Part 2 features a lights-off trial I instigated and documented in Portishead.

  3. […] (This follows on a bit from this post about a traffic junction) […]

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