Wednesday 3 September 2014

A national map of cycling to work

I've recently being doing some visualisation work with the newly released Census commuting data from 2011. I've produced maps of all travel to work, and travel by car, train and bus. I've now done a map of cycling to work (below). This map is particularly interesting in relation to the patterns it reveals but also in relation to the strange long-distance flows we can see. I'm certainly not saying that anyone actually commutes by bike between Manchester and Bristol, as the map may suggest. Click on the big version and have a look around to see if you can spot anything interesting or particularly unexpected. A version with some place name labels can be found here.
This data comes from Question 41 of the 2011 Census form, which asked people to say how they 'usually' travelled to work in relation to the mode of transport which accounted for the largest part, by distance, of their journey. The results can look quite beautiful on a map, but they can also be confusing. Look closely at the map above and you'll ask yourself why there are so many long distance cyclists in England and Wales. More seriously, you might begin to question the validity of the data, the honesty of respondents or some other aspect of the results. 

The ability to interrogate datasets in this way is one of the strengths of visualising large datasets in that we can often immediately identify anomalous patterns or results that confound expectations or are just plain wrong. I'm not entirely sure what's going on with the long-distance flows. Perhaps some people take their bike on a train so ticked the 'bike' option, despite the train journey being longer. Perhaps some people live in one part of the country during the week and cycle to work there but then live at their usual address during the weekend and this is registered as their residence on the Census forms. I'm only speculating but this could be one possible explanation. 

In the image below, I've filtered the data so that only flows of 2 or more are shown. This significantly reduces the visual clutter, but also draws out stronger long distance connections between places such as Bristol and Manchester, and indeed Manchester and lots of other places. Take a closer look by clicking the link below this map. I've added some place names to this map to help with orientation.

Go to the full size version

I'd be keen to hear different interpretations on the data. You get similar results when you map the 'walk to work' data so there's definitely something interesting going on with how people have answered the Census question and the data we have to work with. I'm certainly not saying it's 'wrong', more that we need to understand what exactly it tells us. For now, I'll leave it at that.

N.B. Why didn't I include Scotland and Northern Ireland? The data are not out yet. It's not some ploy to exclude anyone and I know the blog title says 'national' so forgive me if that threw you. I intend to expand the analysis in due course.