8 Feb 2014 21:03 GMT
This week, Londoners were confronted with the challenge of trying to get to and from work with most of the Tube network either suspended or running a reduced service for two days. Whatever your opinion of the strike, it's hard to deny that the impact of closing the Tube is large. While playing around with some population data today, I discovered one of the reasons why.
One of the ways you can quantify how London's population changes during the day is to compare the residential population of London with the workday population. The residential population is based on where people live — it's the conventional measure of the population that is used in surveys and the Census to allocate each person living in the country to the population of a particular area given their home address.
But many people spend most of their waking life outside their home, at their place of work. The workday population is an alternative measure of the population, which is based on where working people work and non-working people live — it's a way of allocating people to the population of an area based on where they spend their working lives if they work, or where they live if they don't have a usual place of work or don't go out to work.
The difference between the residential population and the workday population does not provide a direct measure of the movement of people in and out of London during the day. First of all, the difference between the two populations is the net difference between people moving in and out of London when they go to work; and while most people travel into London to work, some people who live in central London travel out of London to work elsewhere. Second, the workday population includes shift and night workers, who are not at work during the daytime. Third, neither measure of the population includes short-term residents or visitors to London.
Nevertheless, comparing the two populations shows that the workday population is significantly larger than the residential population in the most central parts of London, and the effect is concentrated in a very small area.
This visualisation shows the population density of Parliamentary constituencies in London, based on both their residential and workday populations, as recorded at the 2011 Census. The map alternates between the two measures, and is shaded to represent the density of both populations using the same linear scale, where white is the lowest recorded density in any constituency, and black is the highest.
The map shows that a small number of constituencies in central London have much larger workday populations than residential populations. Based on the 2011 Census, the workday population of the Cities of London and Westminster constituency is around 946,000, which is almost nine times the residential population of 110,000. And because the constituency covers an area of just 16.4 square kilometres, the density of the workday population is around 57,700 people per square kilometre.
Other constituencies with large differences between their workday and residential populations are Holborn and St Pancras (+174,000), Poplar and Limehouse (+97,000), and Islington and Finsbury (+73,000). But there is no other constituency in London whose workday population density is comparable to the Cities of London and Westminster. The constituency population with the next highest density is the workday population of Holborn and St Pancras, which at 23,500 people per square kilometre is less than half as dense.
With so many people trying to get into such a small area at the very heart of London, it's no wonder a Tube strike can be so disruptive.