April 6, 2017 - 3 minutes read

Hi everyone, and welcome to the first in our series of Curious About… In this series we’ll be looking at a series of problems and brainteasers given to us by our fantastic tutors here at Oxbridge International Summer School do you can test out your skills. This week, we have an economics problem that will make you think every time your at a train station!

### A: Let’s assume the station-builders weren’t just crazy, and there are two escalators for a reason.

Escalators transport commuters up and down, so intuitively there must be ~twice as many commuters coming up as going down, or to be more precise ~twice as many people/minute on average. But that can’t be right – on average the same number of commuters go up as down (you’d sure hope everyone who enters the tube leaves it too)!

But that only tells us about average flow – on average, the same number of commuters go up as down. But we don’t only need escalator capacity to accommodate average flow – we really need it to accommodate peak flow. Imagine everybody taking the tube at the same time, rush hour: most of the day the tube is empty, so average flow is low, but we’d need a huge array of escalators to accommodate all these commuters at 5pm. The high peak flow requires a huge escalator capacity.

Why would peak flow be higher coming up than going down? For this we need to get back to basics: imagine the processes of arriving at, and leaving, a station. Commuters arrive from the street at random, in a smooth flow. Minute to minute roughly the same number of commuters arrive.

Commuters arriving from the train, however, land at the same time. Once every ~4 minutes, large numbers of commuters surge from the train to the escalator, creating a huge peak. To accommodate this peak, station planners build two escalators.

You could also draw a graph illustrating the flow of people into and out of the station every minute.

### Why are escalators relevant to economics?

This question is a perfect opportunity to test out the skills you need to be successful in economics, and is great at illustrating some of the key principles. Thinking about the flow of people in and out of a train or tube station is actually quite similar to thinking about the flow of goods and services into and out of countries and firms. This question also gives you a great opportunity to draw a graph. Visualising and simplifying complex problems is key to economics, from thinking about widgets in a firm, to national debt and the skills used by this question are key to this.