Friday, 20 March 2015

Traffic Signal Pie: So Near, Yet So Far

I have covered stand alone signalised crossings before, but an issue popped up over the last few days which is worth airing in its own right.

Our story starts with a video posted by Matt Turner of the Great Gas Beetle blog showing a Puffin crossing within a large signalised junction in Sheffield which had recently been reworked (Streetview variously shows the old layout and works in progress). Before we proceed, watch the video for yourself;

When I first watched it, I couldn't work quite what the problem was, but clearly, people crossing ahead could see a green man shown on the Puffin display - a "near side" display (known as a Pedestrian Display Unit - PDU). The issue is that the green man is actually for the people using the crossing to the left (from where the video starts) with the ahead signal being on the right hand side of the crossing.

Puffin crossings, whether stand alone or within a junction have the PDU above the push button. When a green man is shown, you can cross - my earlier post went into more technical detail as there is pedestrian detection involved too. The push button is normally (but not mandatorily) on the right hand side as you face the crossing which is there for consistency for visually impaired people, but in terms of the PDU, it will normally be placed so it is on the side the traffic is approaching from and they are turned to if one looks at them, one kind of sees the traffic in the background.

In the Sheffield example, the crossing point in question has the PDUs on the approaching traffic side. The side the video is taken has the push button under the display (on the right), but for people crossing back, the PDU is in the left (as you cross) which is on the traffic approach side. For people crossing back, there is a push button on the right (for consistency), but also another on the left. Yes, it does all seem a bit confusing already. 

The video is taken standing on a triangular island from which three crossing emanate, all with various posts, PDUs and push buttons which, on the face of it, are kind of set out correctly with at least a push button on the right and PDUs facing traffic (where displays are on the left, there is also a push button). 

The underlying issue with the layout is that the triangular island is very small and so in terms of the "wrong" green man which is the subject of the video, it is positioned about the same (but mirror image) as the "right" display. Granted it is turned to face traffic, but as that traffic is coming through a central reservation, the "wrong" display is still very easily seen.

The original junction was laid out pretty much the same from what I can make out in Streetview, but the important difference is that it used to have far-sided signals (known as PedX crossings);

In the image above, pink blob and arrow shows where the video was taken. The yellow circles show the push buttons (both on the right) and the green circles show the old far-sided signals. The red circle is the push button for the crossing to the left as shown in the video.

With the old layout, the implication of someone pushing the "wrong" button is that they will never get a green man at the far-sided signal they are looking at. They might think there is a fault and cross in a traffic gap, although given that traffic will be stopped at some point (to let other traffic movements go), a green man would come in automatically. The "wrong" push button might still be an issue for a visually impaired people who may not appreciate the button being on the left. 

With the old layout, the triangular island is still small and so the "wrong" button is still in reach. The junction is very much laid out (was and still is) to maximise the throughput of motorised traffic and as is pretty much always the case with road layouts like this, pedestrians are given several crossings to negotiate, with very small islands to wait on. There are other layout issues, but I will stick to the signals issue.

Of the two layouts, although both poor for pedestrians, the old far-sided signal layout does not create the additional risk of someone mistaking the green man on the wrong signal as theirs. This issue has been debated on Twitter (well so far as you can) and it has been suggested that people should have been taught how to cross and where "their" green man is. I think that is nonsense because we are dealing with people who are fallible and besides, people don't always see official training on such things!

One of the big problems with Puffin crossings is that when they are busy, people not standing immediately next to the display simply cannot see the green man. Imagine this location is busy, some people won't see the red man, but they might see the green and assume they can walk out into live traffic. Where high numbers of people are expected, then a second, high level display can be provided, but it will never be as good as the visibility afforded by a far-sided signal.

There are good reasons to use near-sided signals and that is where far-sided signals could be confusing. The layout on the left is a two stage non-staggered crossing - essentially two separate crossings (Toucans in this case). 

If far-sided signals were used then there is the possibility (perhaps remote in this example) that someone crossing could mistake a green man in the distance on the second crossing as theirs. We can add louvres to the green men (so they can only be seen from the right position), but it is another maintenance issue and they just don't give the same clear view.

This example has a second set of higher level displays, but in reality it never gets that busy and so all in all, I think the right choice has been made in terms of the near-sided signals.

In this next photo (a 2-stage cycle crossing running parallel to a PedX crossing) it is possible to confuse the signal in the distance (circled) red as the signal for the first crossing. There is a signal for those cycling immediately at the crossing point, but it is up on a pole and so you are relying on the secondary signal circled in green. If the far away signal goes green, then again, we have a serious issue. 

The lesson from all of this is that we must always take care that the right signal can only be seen by the right people and this follows for those driving too - there is a phenomenon whereby people's eyes can settle into a particular focus where things further away get noticed before things closer, with signals and PDUs this is called "see through".

What do users think about near side and far side signals? There is some research from 2005 by Transport for London which looked at the views of people using Pelican (far-sided signals) and Puffin (near-sided signals). The conclusion (of quite a detailed study) was;

In summary, Puffin crossings are slightly more likely than Pelican crossings to engender a sense of safety among pedestrians. While neither type of crossing could reasonably be described as presenting any fundamental difficulties of use, Puffin crossings might benefit from a general review of the visibility of pedestrian signals. At some sites, provision of additional signals would solve the problem of obstruction by other pedestrians. Where this is not possible, it will be important to make sure that all Puffin crossings have audible signals as well as visual ones. 

The study was only looking at stand alone crossings; as Pelicans use flashing green men/ amber traffic signals, they are never used at junctions, but it is a useful piece of research in terms of user experience. For a while, TfL was quite interested in using near-side signals for new schemes and upgrades of existing kit and even back then, they were years behind other parts of the country which seemed to have embraced near-sided signals already. Of course, TfL are now more interested in "Countdown" which goes back to far-sided signals with an additional signal which counts down the time left to cross when the green man goes out and before the red man is displayed and they have produced a study showing people like that system too! (a whole other discussion)

The DfT published (now archived) guide to Puffin crossings (near-sided signals). Appendix D sets out some of the other research and the flavour of the document as a whole was in favour of Puffins. Interestingly, there is a section on PDU positioning (p22) and it discusses using "reduced angle of view" PDUs where see through could be an issue. The photo demonstrating the point is from Sheffield City Council! Reduced angle of view is all about the optics in the PDU which can only be see from a narrow angle. From what we understand, Sheffield is looking to rotate the pole of the offending PDU. I am not sure if there is a reduced angle PDU or not, but clearly something needs to be done. I don't know if this will work or not, we shall see.

So, what can we learn from this? For me, it is all about using the right tool for the job and not having a rigid policy one way or another because each site we look at will have its own issues to consider. It is also about checking signal installations at every stage and especially when they have been switched on to make absolutely sure that nobody gets confused. We all need to remember that if there is any confusion, then it is the people outside of the vehicles who will always come off worst.


  1. Maybe it's just my eyes but I generally much prefer the far-sided signals, largely because you don't have to massively re-focus your eyes between the traffic (far) and a near-side PDU which is very close and low down (I'm tall). With far side you can generally keep an eye on traffic and the signal; with near side I find myself glancing down to the PDU and up to the traffic and back. Also near side take your gaze away from where you're about to step, which feels unnatural - could be dangerous if you step out while still focused close and low on the PDU, without first re-orienting your vision to 'up and far' to look along the road for oncoming traffic. With far side it's far easier to just glace a bit to the right once you see teh green man - your eyes are already distance-focused.

    1. I am biased of course, but I agree with you - I have never found nearside to be particularly ergonomic, but they are useful in some locations.

  2. I'm a signals engineer and, in my experience, nearside signals are widely disliked even within the industry. The idea that they encourage pedestrians to look at approaching vehicles and not step out blindly on the green man is misplaced:
    (1) it's up to drivers to obey traffic signals
    (2) it's in pedestrians' best interests to watch for vehicles anyway (including those that may be legimitately proceeding through the green man - stragglers through a junction, emergency vehicles, etc)
    (3) many standalone puffin crossings are fitted with audibles. So pedestrians walk out blindly on the audibles instead of farside signals.

    Additionally, nearside signals IMO cause more problems than they solve:
    (1) As you pointed out, they're terrible for read-through
    (2) They don't work at busy crossings as the view of the signal gets blocked by other pedestrians
    (3) When pedestrians are losing right of way (i.e. green man is changing to red man), they don't work as a "STOP" signal as there is no signal in the line of sight. This leads to conflict when vehicle green appears again.

    Like a lot of DfT guidance (direction of staggered crossings anyone?), it seems to be a recommendation based on a perceived risk rather than an analysis of what pedestrians actually do.

    1. Yes, I think we agree in the main, although in limited locations nearside can be useful - I wouldn't wish to rule them out totally, but nor should there be a firm policy one way or the other - right tool for the job and all that.

  3. The Dutch never have pedestrians waiting in the middle at a zebra or traffic light controlled crossing. Even at crossings where drivers are only encouraged but not required to give way, the speeds are pretty low, 50 km/h or less (there are some pedestrian give way crossings on 70 km/h roads, but these are where there is a roundabout to slow traffic), and with low enough volumes that you frequently find gaps, there is a wide median refuge and the crossing isn't staggered.

    Of course this post is about traffic lights, so let's stay on that subject. Puffin crossings I mostly like, but they require four things in my opinion. A guaranteed low waiting time, no more than 8 seconds, 4-5 seconds preferably (assuming such crossing isn't combined with a signal controlled junction for motor vehicles as well, if that is the case, then this is 40 seconds maximum, and it should be 30 seconds or less if possible). A waiting time indicator really helps people to be reassured that you really are important and that we think of you, and if you are confident that your waiting time is reasonable given the volumes of traffic, then you are much more likely to obey the red man even if you aren't required to. And finally, a far side green man light (and red man) is useful, as it helps people to be assured that they are going to have a protected green man, plus it helps you know which crossing is actually getting the green man. And finally, while there should be median refuges and a call box in the middle if needed when possible, the timings must ensure that you have enough time to cross an entire arm in one go.

    Of course, these crossings should be avoided when possible. Zebra crossings and uncontrolled crossings are superior if the conditions will allow it, and on very large roads, grade separation can be suitable if well designed.

    These are the things that make traffic lights for pedestrians (and the same principles apply to cyclists, except that cyclists would have their own cycle track using the protected junction or simultaneous green) helpful for them and makes it safer.

    1. You do get two stage crossings in the NL.
      Its out of the town centre and not a major bike/walking route. The primary cycle route parallel to this have no traffic lights.

    2. I assume that is a single stage crossing for cyclists and two-stage for pedestrians? Doesn't look particularly nice for people walking!