I have been teaching the MBA core operations class this quarter. This week we just wrapped up talking about bottlenecks and capacity. I consequently found an article in The Guardian rather timely (The tube at a standstill: why TfL stopped people walking up the escalators, Jan 16). TfL in the article title refers to “Transport for London” which runs the Underground. The article reports on an experiment run at their Holborn station.
The experiment in question gets to a bit of escalator etiquette. Specifically, when using an escalator, should people stand to one side so those in a hurry (or in need of a work out) can walk up the escalator or should people patiently stand two abreast? Now if you prefer to chug up the stairs, you clearly lose under the second scenario. However, can it be the case that accommodating the walkers cost the system as a whole an unacceptable amount of capacity?
It’s all very well keeping one side of the escalator clear for people in a rush, but in stations with long, steep walkways, only a small proportion are likely to be willing to climb. In lots of places, with short escalators or minimal congestion, this doesn’t much matter. But a 2002 study of escalator capacity on the Underground found that on machines such as those at Holborn, with a vertical height of 24 metres, only 40% would even contemplate it. By encouraging their preference, TfL effectively halves the capacity of the escalator in question, and creates significantly more crowding below, slowing everyone down.
This graphic gives you an idea of how holding capacity for walkers may hammer throughput.
This is just a fun example to think about. The issue here is not that walkers hurt capacity, per se. Indeed, they should increase capacity. A standing customer occupies a step for the entire time it takes the escalator to make its run from the bottom to the top. However, a walker spends less time on the run and if walkers could be packed as densely as the standers, capacity would go up. But walkers either cannot be packed as tightly (walkers do not tend to move at the same pace a la marching soldiers) or there are just not enough of them. The latter is the problem for the Tube. If there were enough walkers, then the half of capacity dedicated to them (i.e., the left side of every escalator) would be heavily utilized. If there aren’t a sufficient number of walkers, capacity is wasted. (Note that in the US, the TSA has encountered similar problem with its pre-check program.)
As the article tells it, the program was a great success.
An escalator that carried 12,745 customers between 8.30 and 9.30am in a normal week, for example, carried 16,220 when it was designated standing only.
So will standing only become the norm in London Tube stations? Maybe not. There are some implementation issues to work. The test went for three weeks and was implemented with TfL staff standing around with “loudhailers” cajoling commuters to follow the new rules. As soon as the experiment ended, commuters went back to their old ways, standing on the right to accommodate a handful of walkers.