Saturday, 11 January 2014

Traffic Signal Pie - First Slice

We have all heard of traffic jam, but what about traffic signal pie? This week I will try and explain how traffic signals work at stand-alone crossings (bear with me as I am bound to get something wrong!)

Traffic signal engineering is a whole sub-set of highway engineering and the people who make it work are clever people. Much of the technology is beyond me, but I will do my best to give you some insight over the next few weeks (I am not sure how many posts this will take and if it will be signals every week for a bit yet - yes, making it up as I go along). When talking about "drivers", this is defined in legislation for the purposes of traffic signals and will apply to those using the carriageway - including cyclists! (it is weird, but rules is rules).

Some other useful lingo is that we refer to traffic signals rather than traffic lights. When we talk about each individual "light", we call them "aspects". For example, a basic traffic signal aimed at "drivers" has three aspects; red, amber and green. The posts on which the traffic signals sit are called poles and the cluster of signals is known as a head.

A Pelican crossing (red man way in the distance) and the ubiquitous
push button to give pedestrian demand.
Let's start with standalone crossings. We have Pelican, Puffin, Toucan and Pegasus. Pelican gets its name from Pedestrian LIght CONtrolled (PELICON - it's artistic licence). Essentially, pedestrians arrive at the crossing and see a "red man" on the other side of the road. They press a button on a yellow box on a pole on their side of the road to "tell" the signal computer that they want to cross (demand). The box lights up with a "WAIT" signal to show pedestrians that the button has been pressed) - #streetgeek time. As part of a recent consultation on changes traffic signs, it was suggested that the "WAIT" be replaced with a red light similar for Puffins (see below) because of a lack of supply of the filament bulbs for the "WAIT"!

After a bit drivers get an amber signal and then quickly, a red traffic signal. Shortly after pedestrians get a steady "green man" which appears on the traffic signals on the other side of the carriageway. After a while the green man flashes and then just after, drivers get a flashing amber and then a green.

The amber and then red signals to drivers is an absolute instruction to stop. The flashing amber is an instruction to allow people crossing to finish. The green is an indication that drivers may proceed beyond the stop line. The red man is essentially a warning that it is not safe to cross. The steady green man suggests that pedestrians may cross. The flashing green man means that a pedestrian may finish crossing, but shouldn't start to cross if not already started.

A Toucan crossing with far side aspects. Under the red man is the
green man and to the right of the green man is the green bike.
Next up, Toucan crossings. Toucan is really silly "Two Can" cross which means both pedestrians and cyclists. Toucans are basically the same as Pelicans, except there is a green bike as well as a green man (more on this in a bit) and there is no flashing amber/ green. When the bike/ pedestrian stage is complete, drivers will see a red, red/amber, then green. To indicate demand, cyclists press the button on the yellow box on the pole.

A Puffin crossing. You can just see the red man on the push button
box. In this case, there are extra pedestrian aspects as this crossing
is often busy and people block the view of the lower box. Puffins tend
to have two push buttons on each side of the road.
Puffins continue the avian theme and this is shorthand for Pedestrian User-Friendly Intelligent Crossing. The big difference is that the pedestrian aspects are displayed on the push-button (near side aspects) whereas for a Pelican, they are far-sided. The pedestrian aspects are the simple red/ green men. When the button is pressed, a little red rectangle lights up or a ring of red lights around the push button lights up. The signal phasing is the same for a Toucan (no flashing amber/ green).

The "Countdown" aspect. This one is actually outside TfL HQ in
Southwark, London on a junction, but they are being used at
standalone crossings too.
Just to get a bit more confusing, Toucans can also be like Puffins and have near side aspects and in this case there is a red bike as well as a red man. It is all getting rather messy. The other thing to cover now is the non-birdlike Pedestrian Countdown At Traffic Signals (PC@TS). 

Pushed by Transport for London (and now with UK-wide authority for use by the Department for Transport), this system adds another pedestrian aspect on Pelican crossings (and junctions which I will cover another time). PC@TS essentially creates a hybrid Pelican where the flashing amber/ green man is replaced. Drivers will get the red, red/ amber, green sequence and pedestrians will see a green man and the the countdown aspect which is an amber signal which counts down the number of seconds in which they have left to cross. There is a very small gap between the end of the countdown and traffic getting a green.

The final type of signalised crossing is a Pegasus which has nothing to do with birds, it is for people on horseback. From memory, there are 4 in London and they are basically Toucans for riders and horses with a high level push button for the rider. They can have far or near sided aspects and red/ green pictures of someone on a horse is used.

I have already mentioned pedestrians and cyclists pushing a button to indicate crossing demand, but what other detection is used with crossings? For traffic, the basic arrangement is the use of microwave vehicle detectors (MVDs) which are the little camera-like boxes on top of the traffic signals. They essentially emit a beam which a moving vehicle causes to reflect back to the unit and is so detected. The other method of detection is magnetic induction loops cut into the carriageway surface. Where general traffic speeds are above 35mph, then additional sets of loops are provided further away from the crossing to track the approach speed over distance which is a safety feature.

For Puffin crossings and "Puffin-style" Toucans (near sided aspects), "people" detection is used. First, sensors can "see" people waiting to cross and so if the demand button is pressed and someone wanders off, the demand can be cancelled, thus not triggering a red signal to traffic. They also have sensors "watching" the crossing area and if there are people taking longer to cross (because there are lots of them, or someone is moving more slowly), then traffic will be held a little longer.

I have no idea how these sensors work and so if you do, then please add a comment! I do know that some of the older kit was a bit temperamental and used to pick up things other than people which messed up the timings. There also used to be pressure mats in the footway to detect someone waiting, but I don't think these are used any more because again, they were temperamental.

Not the Vogons as suggested by some on Twitter, but a tactile cone.
A little out of focus, but you get the point!
There are two other features to mention. First, signalised crossings can have an audible (bleeper) signal which runs when the green man is shown to assist blind and partially-sighted people. Sometimes it turns off or the volume reduces at night if used in a residential area.

Second, there is the "tactile cone" which is a little upside down cone placed under the push button of the crossing. The cone rotates where then green man shows and again, is to help blind and partially sighted people, but will be of assistance to anyone with reduced hearing and eyesight.

So, what about the timings and the set-up of a signalised crossing. If you are really interested in the detail there is loads in The Design of Pedestrian Crossings, but I want to keep things a little more simple.

In the diagram, I have amalgamated some of the information from the design guide linked above, but added little pictures of who sees what because I think it is easier to follow.

The reason I am calling these posts "traffic signal pie" is because it is a useful way to think of how time is allocated to who gets which signal. For a Pelican crossing, there are 7 slices which can vary in time, but for any given situation will add up to the whole pie - the whole cycle of the signals. For a pelican crossing, of course, the flashing amber is for the pedestrians, but if they are finished crossing, the drivers can have what is left.

The time ranges for the periods in the diagram depend on site conditions and the design guide gives full details. (A) is all to do with traffic flow, speed and method of detection. If there is no pedestrian demand, the signal to traffic stays green. When the button is pressed, traffic might be stopped quickly if it has had a green for some time. A road with faster traffic (actual speeds are used which is a debate in itself) will have a longer period before stopping so approach speeds can be checked and loops are used.

(B) is fixed and allows a driver to clear the crossing on the amber signal if they are too close to stop. (C) is set to 3 seconds where the speed is above 35mph (read the guidance for more). (D) varies by crossing width (it is in the guidance and for example is set to 4 seconds for a crossing of up to 7.5 metres). The plus 2 is used if it is observed that people turn back thinking they cannot cross in time.

(E) can be used to extend the red signal to drivers if there is concern or evidence that drivers are perhaps bullying their way through on a flashing amber. (F) is normally set to 6 seconds, plus 1 second for each 1.2 metres in crossing width above 6 metres. Periods (D), (E) and (F) are essentially the part of the cycle which campaigners for longer crossing times are targeting. Period (G) is set at 1 second for crossings under 10.5 metres wide and 2 for wider crossings.

A slightly non-standard layout here. There are far-sided traffic signals
and the green is a bike symbol. This is not a Toucan, but actually a
junction - there is a stop line for bikes just out of shot although in this
case, demand is still push buttons.
The other types of crossing have similar set ups, but obviously without the flashing amber. This is a long post and so I will leave you to look up those in the guidance. One point to mention is that we haven't made out minds up on what is the best form of signalised crossing. In London, TfL did studies on Puffins and Countdown and both were generally positive. In fact, TfL are pushing for Countdown (with far-sided signals) everywhere now. There has been controversy as TfL has used the opportunity to reduce the time given to the green man period.

Personally as a user, I like far-sided signals with Countdown (i.e. no flashing amber) on the basis that I can see when I can cross and I know how long I have left and I hope authorisation is given to allow it at Toucans too. The timing issue is important, but not the point of this post. The issue I have with Puffins is that once you start to cross, you have lost the "comfort" of seeing a signal. The flip side was that Puffins were set up to you look towards traffic when watching for a green man and the detection gave you longer if needed.

A far-sided set up with detection is theoretically possible in that walking speed can be measured during the green man period and a slower person would trigger a higher starting number for Countdown. I don't think anyone is looking at this unfortunately. It is worth mentioning the tricky subject of high speed roads and that is anything 40mph and higher. Detection using loops is absolutely vital for speeds (actual) above 35mph and up 50mph. Beyond that, serious consideration should be given to reducing traffic speeds (see page 3 in the linked document) because of the clear safety risks to pedestrians if a driver ignores the signal (OK, bad enough at lower speeds). Actually, a bridge or tunnel might be a better idea.

That's it for this post. I will debate time allocation in a future post, but hopefully this gives you a start in the tricky issue of signals.


  1. When a pedestrian I hate Puffin crossings- for that very reason of the red/green man box being near side, so you have to stand neck craned to the side instead of looking across to where you are going to walk.

    the pedestrian "detectors" often seem to be very focused on the spot right by the button too, if you move away then it resets. Very un-userfriendly.

    1. Some of it is what you were brought up with - I have never been fully happy with near-sided signals, but some people prefer them. The detection needs to be set up properly and checked like any other kit.

  2. In Cambridge we have crossings that are, for bikes, signalled as junctions- three aspect heads and using magnetic induction loops ( and some others that are simply bike-only arms of junctions)

  3. It seems everywhere round here (NE Fife, Dundee) are swapping pelicans for puffins. I find them so annoying for the same reasons as jrg, and annoying as a driver, as you end up having to sit at a red light for no reason when people have finished crossing instead of getting a flashing amber.

    I also think that isolated pedestrian crossing that are not part of a junction should change immediately to the green man instead of having to wait minutes.

    1. Of course, there is a flip side. There are many aggressive drivers out there and bully people who are still crossing when the amber of a pelican starts to flash.

      If set up properly, there shouldn't be an excessive wait for peds. It is often perceptive because if you arrive at the crossing just as the red man comes on, you might be waiting up to a minute for a green man again on the basis that traffic is being let through.

      I had a scheme with a Toucan where period A was 35 seconds, but pedestrians/ cyclists crossed regularly and it caused complaints from drivers about "constant" signal changes. The trouble was that at peak times the traffic was stuffed anyway and all you could see as a driver was being being allowed across the road.

      In this case, A was changed to 40 seconds which was a little better for drivers, but still with the odd complaint by pedestrians. At this stage, I think it is getting towards a political decision on who gets the pie!

    2. Thank you for this post! I had thought I was going craaazy a few nights waiting for the lights to change at a toucan near mine, only to look down and see that the button 'hadn't' been pressed. Instead the sensor had decided I wasn't sufficiently present so didn't warrant slowing the traffic at all.

      At this same crossing, often populated by lots of peds and cycles and ostensibly on a cycle 'superhighway', I have waited and timed it, and the traffic gets the maximum 60 seconds, and all other users get about 8 if I remember correctly. And it does seem to regularly take 60 seconds to change, regardless if there were users present for the previously 'skipped' cycle.

      It's signals like this one that really 'drive' home just how low down the hierarchy of users pedestrians and cyclists are, regardless of what it says in the version that councils publish.

  4. "The final type of signalised crossing is a Pegasus which has nothing to do with birds..."
    But Pegasus did have wings, so broadly in keeping with the theme.

    "I also think that isolated pedestrian crossing that are not part of a junction should change immediately to the green man instead of having to wait minutes."
    If you are waiting 'minutes' for a standalone crossing to change to green then contact the local highway authority. It should be seconds. Having said that people (engineers included) are notoriously bad at estimating distances, speeds and times. Tell them that, as drivers, the maximum time they should ever have to wait to get a green is two minutes and they'll call you a liar, it feels like five.


  5. "the pedestrian "detectors" often seem to be very focused on the spot right by the button too, if you move away then it resets. Very un-userfriendly."
    "4.6 Pedestrian Kerbside Detection
    It is essential that kerbside pedestrian detection
    holds the demand when pedestrians are present
    but cancels the demand if all waiting pedestrians
    cross in gaps. Kerbside detection using different
    technologies may be installed in-ground or above
    ground. Care should be taken to ensure
    sufficient detectors are used to cover the full
    pedestrian waiting area. Detailed information
    should be sought from suppliers on the size of the
    detection zone of their detectors so that the
    number of detectors required may be calculated."
    If you feel there is a fault with a signalised crossing, again, contact the local highway authority. These facilities are put in, at no little expense, to give peds, cyclists and equestrians safe crossing opportunities they would not otherwise have. Any time you think there's a fault it should be reported. If the LA don't know something's gone wrong in the first place, then it can't be fixed.


  6. Pegasus and wings - OK, fair enough!

    Good comments here!

  7. Sadly, I've often wondered what we'd call a cyclist only crossing and keep the flying theme going. Only thing I could think of was an Elliot or ET, maybe, for those in the know, a heron?


    1. Perhaps Guy Martin showed as this week on his flying bike!

    2. Maybe a certain bike shop could sponsor them to be Condors?

  8. An interesting point I learned a few months ago is that if the loop detectors spot a car travelling towards the crossing at above the speed limit the lights will be prevented from changing as a 'safety feature'. So speeding is effectively rewarded for motor traffic!

    1. That seems very unfair. In Spain they have traffic signals that turn red if they detect that the approaching vehicle is speeding.

  9. "In Spain they have traffic signals that turn red if they detect that the approaching vehicle is speeding."

    I've no problem with us taking ideas form overseas, but I'd rather we took them from countries with equal or better accident records than us. Until recently Spain didn't even supply accident data to EuroRAP, whether because it just wasn't recorded or out of sheer embarrassment I don't know.


    1. I'm not actually suggesting we put in stand alone signals as a speed control measure (just stick in a speed camera and be done with it). I was using it as a way of illustrating how ridiculous our current way of rewarding bad driving is.

    2. Cameras: now that's a non-political solution!

      I have no particular issue with them, but I prefer average speed.

  10. I wonder if, when writing about signals and junctions, it's better to refer to 'pedestrian traffic', 'motor traffic', 'cycle traffic', rather than 'traffic', 'pedestrians' and 'cyclists' - if only to start to convince tfl and the boroughs to interpret the TMA (2004) as it was intended, rather than only as applying to motor vehicles?

  11. There is a Toucan using a flashing amber in Southwark - sure its a mistake but its there. I always thought it would be best to give peds a indication when the lights will next change for them rather than telling them how long left to cross. But that wouldn't help speed peds across the road therefore 'smoothing the traffic flow'. This type of light is used on the continent and could stop people crossing on the red man - sometimes you think you have waited long enough so walk half way...

  12. Puffin-style nearside indicators for pedestrian traffic cannot be seen when someone is standing in front of them so are unsuitable when more than one pedestrian is present at a time. They require a 180-degree head turn to view vehicle traffic coming from the left so are unsuitable for two-way single carriageways. There is no signal visible to the pedestrian during crossing so are unsuitable for jurisdictions where jaywalking is not illegal. Pelicans have none of these drawbacks and can be further improved by countdown indicators and nearside repeaters. Puffins are a disaster. Joe, May14

  13. Pedestrian and/or vehicle traffic detection could be incorporated in any type of crossing, so the supposed advantage of the Puffin in having this is spurious. A Pelican could be fitted with this or any other feature already sported by the Puffin. The crosswalk incorporating an arrow with a vibrating bar in the request fitting as used in parts of Australia, the US, Eire and many others is more user-friendly than anything in the UK. Joe, May14

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  15. You might be wondering Ranty how this doesn't cause a problem to the flow of motor traffic. Think about who is going to be caught in the red light. Some driver. Does it matter to the flow which driver? Not really. Buses and emergency vehicles, yeh those can get priority, but the average vehicle. At some point the red light will always affect some driver at some point, so it doesn't really matter when it's called. But for cyclists and pedestrians, long waiting times are going to deter them, and thus are less likely to walk or ride, or do it less often, and this is a bad thing, so the cycle waiting times are as short as possible.

  16. I think that I would prefer a puffin crossing, but I have two main things that I would change. First, I would add a far side signal that shows the same indication as the eye level lights. And second, add a waiting time indicator so I know how long my wait time will be. These are the only two that would be universal across all puffins from the physical design, the programming and design at individual puffins will vary.