Works to remodel the Old Street roundabout are nearing completion, above ground at least. It was still possible to drive round it five years ago, then massive sequential roadworks intervened and the northwest arm was slowly pedestrianised. Cars now have to negotiate a very different streetscape, in some cases involving a lot more traffic lights, in much the same way as Highbury Corner was de-gyratoried a few years earlier.
A new exit from the station has recently opened in what used to be the middle of the roundabout, with direct pedestrian access to the north and the west. This new piazza has yet to open in its entirety, and the main staircase leading up from the ticket hall isn't yet full width. But whereas the message to pedestrians at Old Street used to be USE THIS SUBWAY it's now very much USE THIS CROSSING, indeed there are several places where pushing a button can now bring the entire junction to a halt.
One of the crossings, on what used to be the northeastern side of the roundabout, is particularly extensive. It crosses a bike lane and two lanes of northbound traffic, then a pedestrian refuge, then three lanes of southbound traffic and another bike lane. And all of this is controlled by a single signal, i.e. seven lanes straight across all in one go, which is exceptionally wide as pedestrian crossings go. All of which means that when the countdown timer kicks in it kicks off at an unusually high number, namely 19, meaning you still have 19 seconds to go before you need to be on the far side.
And what I wondered, obviously, is
Is 19 the highest number used at a Countdown crossing in London?
The obvious place to check next was the big crossing on the southeastern side of the ex-roundabout, officially on City Road. This crosses six lanes rather than seven but is still pretty massive as pedestrian crossings go, so I wasn't sure what number would come up when I pressed the button.
This time the Countdown started at 17, so a much larger number than usual but not a record breaker, suggesting this crossing wasn't quite as wide as the other one. And it turns out crossing width is a crucial factor in determining maximum Countdown time.
The maths of signal timings is set out in Section 6 of the DfT's Traffic Signs Manual, specifically subsection 18.4 (Signal operation). At a countdown crossing the traffic light sequence has seven phases, the first being the lengthy period when vehicles see a green light. Everything else is known as the 'intergreen', this being the entire period the red or amber light is showing. Only in the fourth phase, known as the 'invitation to cross', do pedestrians see a green man. And it's during the fifth phase, which would otherwise be a 'blackout', that a countdown timer counts down.
Hey presto we have a formula for calculating how long a countdown should last and it is indeed "dependent on width of crossing", as well as "chosen design walking speed".
time = L/s where L = width of crossing in m, and s = chosen design walking speed, either 1.0 m/s or 1.2 m/s
For a maximum countdown we'd need the slower of the two walking speeds, which conveniently is 1.0, the easiest possible number to do maths with. It makes the formula ridiculously simple
time = width of crossing in m
i.e. a 5m crossing needs at most a 5 second countdown and a 10m crossing needs at most a 10 second countdown. The mega crossing at Old Street should therefore be 19m wide (or, if a faster 1.2 m/s walking speed was used, 23m wide instead).
Armed with confirmation that the longest countdown timers are at the widest crossings, I headed out to try to find London's ultimate.
Bow Road has lots of pedestrian crossings and I checked them all. The briefest countdown was 3 seconds coming off the Bow Roundabout, which is the traversing of a single lane of traffic. I found ten crossings with a countdown of 4 or 5 seconds, these generally staggered, and a big 9 seconder at the foot of Fairfield Road. But the winner here is at the very western end of Bow Road, a giant 'all the way across' crossing which requires a lofty light above the centre of the road to tame the traffic. It counts down from 12 seconds, which is pretty long but definitely not a winner.
I went to Hyde Park Corner because I knew the crossing between Constitution Hill and Wellington Arch was a whopper, and so it proved when the countdown kicked off with 15.
I went to the Mall because not only is that very wide but I remembered it had some especially dawdly pedestrian crossings. The two at either end both start counting down from 14 but the winner is the crossing in the middle, closest to St James's Palace, which merits a whopping 16.
The Mall is the same width all along but I think what makes the difference here is the junction with Marlborough Road. Incidentally the crossing at the western end, nearest to the palace, has an incredibly long 18 second green man phase before the 14 second countdown begins. These combine to make what may be a record-breaking 32 seconds for a single standalone crossing, likely the result of there being very little traffic and a lot of slow, unfamiliar tourists.
But the longest countdown, I think, is at the fairly obvious location of Oxford Circus. Its Shibuya-style diagonal crossing was introduced in 2009, and diagonals are obviously a lot wider than going straight across so require a longer countdown. I waited patiently for the magic number to appear and boom - it was 20 - and I think we have our winner.
As a quick check, a newspaper article from 2009 confirms that the diagonals at Oxford Circus are 25m long. Applying the official formula and using the faster 1.2 m/s walking speed we get a time of 25 ÷ 1.2 = 20.8, so a 20 second countdown fits pretty well.
It may well be that a wider countdown crossing exists somewhere in London and I haven't thought to visit it. You may know of one, or you may suspect you know of one without actually knowing what the maximum number is, so it'd be good if you could check. But in the absence of evidence to the contrary I reckon these are the pedestrian crossings with London's longest countdowns.
