diamond geezer

 Thursday, October 08, 2009

I think both red and green simultaneously is impossibleHave you been experimented on lately? If you were walking around the capital earlier in the year, you might have been. TfL have been experimenting on Londoners to see what would happen if traffic lights were rephased to give pedestrians crossing the road less time and vehicles more. If you live or work in Wandsworth, Lambeth or the City, you might unwittingly have been part of it.

Boris wants to improve traffic flow in London, and one way to do this would be to increase the proportion of time that traffic lights are on green. It seems that many of London's traffic lights have been over-generous to pedestrians by allowing them too long to start crossing. At many road junctions the green man symbol is illuminated for up to 10 seconds, whereas Government guidelines require only 6. Cut the green man period and, hey presto, four seconds extra for cars. These extra few seconds soon add up, and so traffic flow is improved.

Before you start worrying about safety, it's not crossing times that are being cut, it's starting-to-cross times. The gap between the green man disappearing and the red man lighting up, which is supposed to give anyone on the crossing sufficient time to get to the other side, remains exactly the same.

Here's how the new proposals might work for a typical crossing where one complete traffic light cycle lasts 80 seconds.

green10 sec 
blank10 sec 
red60 sec
green6 sec
blank10 sec
red64 sec

Under the new proposals, traffic would be moving for 80% of the time, not just 75%. Hurrah! For pedestrians, however, green-man-time would be almost halved. Turn up at a normal crossing on foot now and there's 1 in 8 chance that the green man is lit. Rephase the lights, and that falls to a miserable 1 in 13. Boo!

TfL has commissioned research (which you can read in full here, or in summary here) to explore what the implications of greater vehicle friendliness might be. Their experiments were carried out by tweaking timings at nine sets of traffic lights around London. Thousands of pedestrians were then filmed, and observed, and categorised, even interviewed. And some rather obvious conclusions were drawn...

There was a significant increase in vehicle throughput after the signal re-timing.
According to the experiment, vehicle throughput increased by 6.5%. In other words, for every 15 vehicles that passed the lights before, one more slipped through during the extended green-light period. But that's exactly what you'd expect to happen when 'green time' is increased by about 6.5%. It's not rocket science.

pelican crossing, Oxford CircusMore people crossed in the red man phase after the signals were re-timed.
Again, that's obvious. If the red man phase is longer, more pedestrians will cross during it. In the survey, more than half of the pedestrians observed crossed on red, and less than 40% on green. Londoners are already dreadful at waiting for the green man before they start to cross, preferring to nip across when it looks safe or just to rush headlong in the hope that oncoming traffic will stop. Rephase the lights, and even more of us will choose to cross dangerously.

There was no significant effect on safety from the re-timing of the signals.
Recorded observations of traffic flow suggested a statistically significant increase in the number of times road users needed to manoeuvre out of each other's way. Pedestrians deviating from their original route as they crossed, or vehicles undertaking controlled braking, both of these increased. But more serious traffic incidents didn't vary much. Nobody died. Nothing to worry about, then.

Pedestrians with limited mobility felt rushed at crossings, and uneasy crossing the road when the blackout period started.
Most of us can cross four feet of road in one second, which is the baseline standard on which pelican crossing timings are based. But less mobile folk can't necessarily make it across before the red man appears, and a reduced green phase would only make them less willing to try. If anything stops this proposed change from being implemented, it'll be the needs of the elderly and the disabled.

When asked 'What do you think this blackout period means?' 60% of pedestrians either answered incorrectly or didn't know.
It's amazing how many people don't know why the green man disappears. He used to flash, and this blackout means exactly the same thing - "you have plenty of time to continue to cross, but don't start out now". Indeed, so baffled are London's walkers by the 'dark phase' that Boris is considering introducing Countdown timers, like they have in certain American cities, to indicate how long it is until the red man appears. 10, 9 (plenty of time), 8, 7, 6 (if you run, you might get across), 5, 4, 3 (hurry up now!), 2 (hurry!), 1 (hurry!!), 0 (oh bugger here comes the traffic). Expensive to install, especially in these times of TfL austerity, but the additional information might be a real boon for street-crossing risk-takers.

Pedestrians did not notice a change in the junction operation and most felt safe before and after the signal timing changes.
After the green man was cut there was a small drop in the number of pedestrians feeling unsafe, but nothing statistically significant. It seems most people never spotted they were being experimented on at all. So if Boris ever decides to increase car-time and decrease crossing-time, London's pedestrians probably wouldn't notice the additional inconvenience. I bet he does.

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