Saturday, 10 March 2018

Traffic Signal Pie: Just Rephase the Lights

I don't hear this every day, but "just rephase the lights" has become a shorthand for people who cannot see or refuse to see that they are part of the problem.

First, let's get the terminology right and I have the irrepressible Brian Deegan to thank for this wonderful three word description of how traffic signals are arranged;

"Phases make stages"

Phases come from the electronic set up of the signal control computer, essentially they refer to where traffic (drivers or cyclists) or pedestrians are are controlled through a junction on a predictable path with the control being the signals themselves. With pedestrian crossings and situations where opposing traffic flows always run together, then they are taken as single phases.

A stage is where non-conflicting phases can move together and may or may not be used each time in the signal cycle; with the signal cycle itself being basically where every called stage has had a turn (in a preset order). For the very geeky, a more detailed (and slightly more correct) explanation is available on a Traffic Advisory Leaflet here.

So, when people say "just rephase the lights" they probably mean they want the set up changed to favour a particular stage they have an issue with (the one they feel held up on). Rephasing would essentially come from a physical change to the layout because we are dealing with fixed paths or movements. Perhaps they do know more and think that a dedicated phase should be created for them and that it should always be green!

I am not entirely sure how the phrase has come into common use, but at the root of it is a dissatisfaction with how signalised junctions operate (and I never hear the phrase used with standalone signalised crossings). As the running title of this occasional series of posts suggests, we are dealing with something finite and as with a pie, it has to be divided up somehow. My generalised complainant just wants a larger slice.

The size of the pie is governed lots of variables which go back to physical space, the number of traffic lanes, pedestrian demands, cyclists demands, flows changing throughout the day, type of vehicles passing through etc. In essence, for any given situation there will be an optimum pie size. If we are to efficiently consume the pie, we will be able to scoff 90% of it easily without any problems. The last 10% is where people start fighting over the crumbs and it's not efficiently eaten.

Paradoxically, we can sometimes eat more than 100% of the pie (and to be honest it's where my analogy turns a bit suspect), but its people licking the enamel off the plate. The amount which the capacity is utilised is known as the Degree of Saturation (DoS) and so 90% DoS is generally the most efficient situation and as we reach (or sometimes exceed) 100% DoS we are in the realms of people taking shorter gaps, tailgating, jumping amber or red signals etc.

In busy urban places, junctions regularly run above 90% and in many cases, they regularly run beyond 100% (perhaps up to 105% as a rough idea - "over-saturated" if you will). At this stage, the flows completely break down and we see traffic queues quickly forming and if things don't start to clear, these can start to extend exponentially (in theory). We see drivers blocking crossings and other movements as they try to get through the green signals, despite not having clear exits. Junctions with yellow boxes will generally be running beyond 90% because of the need to try and keep drivers from locking up the junction.

Those who are frustrated with the operation of signals rarely want to face up to the reason why the pie has all gone. People do sometimes suggest tweaks that the experts have missed (and signals people are very clever), but in general they don't want to face up to the reality that the congestion is caused by too many people wanting to get through a junction at the same time and that everyone else's journey is less important than their own.

It is interesting (for me at least) to read transport assessments (TAs) for new developments where signalised junctions are impacted. I have often said that I have never seen a TA that admits a scheme will cause congestion, but that has changed recently. I don't know if it is a shift in presentation or a realisation from my TA-producing colleagues, but the arguments being put forward have subtly changed.

What seems to be happening is that more is being made of national forecasting data (i.e. traffic is growing) and where junctions are over-saturated, the argument is being advanced that although queues do in theory grow exponentially, people will change their behaviour and travel at different times, using different modes or not travel at all. They are not quite admitting the development will stuff the signals, they are saying that the junction is either stuffed now or it will be soon, so the development doesn't really change the inevitable.

I don't think this is people throwing up their hands and giving up, but there is a realisation that we can't built ourselves out of the problem - well we could, but we would have to start demolishing our town centres to make their junctions larger. In many cases we did just that to create gyratories which now have had their capacity used up.

Just rephase the lights is an easy opt-out for the public who think there are cheap and simple solutions to our congestion problems. The politics of it doesn't seem to explain the difficulty there is because it is often also too simplistic and not wanting to face up to the challenges. Whether this is because we don't want to be honest with people or whether professionals aren't very good as explaining complex issues plainly, but our problems go way further than those which can be solved by a traffic signal engineer fiddling with traffic signals to give a couple of seconds more green time.


  1. A typically knowledgeable blog thanks.

    I think when Joe Public says 'rephase the lights' there is more to it than sprinkling some magic new timing. I think it can be read to mean the design of lights we have in the UK is inefficient. There are simple changes which could speed all users up.

    A small example from this morning's ride. I was southbound on Chalk Farm Road, moved into the right turning filter lane (to go to Morrisons) I was held at a red light. The southbound and northbound traffic had green. There was no oncoming (northbound) against me. I sat there at a red light for no reason (no I don't jump lights, however tempting this one was). Inevitably, just as a train of northbound vehicles was approaching, the lights changed, they were stopped, and I got a green right turn arrow.,-0.1500521,3a,58.5y,99.4h,87.73t/data=!3m6!1e1!3m4!1sbC6vUjBVuaVoDd014OqoDQ!2e0!7i13312!8i6656

    If that right turn could simply be an amber, proceed at caution, I could have gone earlier, saving me time, and saving a whole train of vehicles from needing to stop.

    If introducing an amber phase is beyond UK regulations, then simply having more interaction between sensors and the lights could have prevented this double hold up (me delayed, and the oncoming traffic delayed).

    1. or alternatively if it had been running on MOVA......

    2. Well yes, some are still set up badly or have old control! No turning on amber in the UK I'm afraid, although Turning The Corner could bring us in line with many other countries