From mega-regions to mega commutes: US commuting working paper

My previous post provided some images from a recent piece of work I did on mapping tract-to-tract commuting patterns in the contiguous United States. This post provides a bit more background and extracts from a working paper, plus some of the original map outputs from the project - which are different in style (kind of a night time view). The focus is also more on mega commutes and mega-regions (think Gottmann's 'megalopolis'). I also provide a bit more detail on the method and data.

A constellation of cities in the Midwest

Being a member of the Regional Studies Association for a good few years now, I've followed various debates about regions, city-regions and mega-regions - including the very interesting work on mega-regions by the America2050 project of the Regional Plan Association. I also have a longstanding interest in commuting flows (and mapping them) so I set myself the challenge of mapping micro-level commuting flows in the contiguous United States in the hope of identifying what I expected would be some interesting mega-region commutes. I also hoped, in the context of this data, that I would discover some of the mega commutes identified by Rapino and Fields of the US Census Bureau. On both counts I wasn't disappointed. The first map below shows the entirety of the lower 48 states and the commuting patterns come out quite clearly.

Journeys to work in the contiguous United States

Obviously, some areas are more interesting than others, so I zoomed in on various areas, including California and the Northeastern United States. The map below shows travel to work patterns in California, and you can clearly see the wider Los Angeles metro area as one large commuter region, the Bay Area as another (but more polycentric), and also the settlement and journey to work patterns in the Central Valley, from Redding in the north down to Bakersfield in the South. This shows the urban settlement patterns in the state of California, but also the spatial configuration of the commuting connections between places.

If you take a closer look at the working paper behind these maps you'll find out more about the data. What I found most interesting were the locations where 'mega commuting' was prevalent, so I looked at the top 20 Census tracts in the Northeastern US with the highest number of people commuting there - i.e. over 50 miles each way. As you can see from the table below, this is dominated by New York City, but Washington DC also features. The total volume may not seem much, but remember that these are quite small Census tracts, with only a few thousand people.

Mega commuting in the Northeastern United States

I then did something slightly different - I wanted to filter the data in a more scientific manner. Since the data provided by US Census Bureau includes a margin of error (MOE) value for each individual tract-to-tract flow, I calculated the coefficient of variation for each individual flow line (there were just over 4 million). These were based on a 90% confidence level, so the formula was simply:

((MOE/1.645)/Commuting Estimate) x 100

I used a rather generous cut-off and then displayed only those flows which had a coefficient of variation of less than 40. The results are shown in the map below. We can see the expected pattern of commuting but - hold on a minute - what are those really long distance lines? Surely people don't 'commute' vast distances like this. Well, it turns out that this might actually be true because many of these lines begin and end in military locations or other places associated with regular, long distance moves for work and since the American Community Survey asks respondents how they usually got to work ‘last week’, it's entirely plausible that a number of people will work away from home and that this will lead to the kinds of patterns we see below. Or, to put it another way, don't think of these long lines as journeys people travel every day! 

If you want to read more about it, you can click below to see the working paper, which also includes links to high resolution versions of the images shown here.

American Commute: working paper

Mapping the American Commute

Update, 20 September 2015: scroll to the bottom of the post if you want to download the data.

One of my summer projects this year has been attempting to map the American commute, following earlier work on a similar subject. Put simply, I've attempted to put together a map which shows commuting connections between locations in the contiguous United States, using the most fine-grained data I could find. Some of the results of this went into a recent piece in WIRED, and also CityMetric, and the larger piece of work it's based on is part of on-going research into the best ways of mapping commuting flows. The main images are below, followed by some more technical information. For now, all you need to know is that these images show commuting connections of 100 miles or less between Census tracts in the lower 48 states. You'll have to forgive me if your city isn't labelled! 

Higher resolution image available here

And now some zoomed in versions...

Zoom in of the west coast

Texas, and beyond!

Interesting patterns of connectivity in the Midwest

Look closely for some interesting inter-connections

The famous BosWash megalopolis

But this just shows where people live, doesn't it? Yes it does. But it also shows how the places where people live connect with other places from a functional economic point of view, at a fairly fine-grained level. It offers a slightly different view than just looking at the urban fabric alone which, I might add, is interesting in itself. Mapping flows like this is not exactly new, as this paper from Arthur Robinson (1955) on Henry Drury Harness (1837) demonstrates. Nonetheless, I haven't seen anyone map travel to work at this resolution for the United States, so I thought I'd have a go myself. 

If you spend some time looking at the big version of the map you can begin to see how places connect and where there are obvious disconnections, even between places that are not that far apart. One thing that you can pick up from the complete dataset (but not this batch of maps) is the growth of mega-commuting, as explained by Melanie Rapino and Alison Fields of the United States Census Bureau. 

Background information: the data I used is the most recent tract-to-tract journey to work dataset from the American Community Survey. This dataset covers journeys to work between the c74,000 census tracts in the United States and the complete dataset has around 4million interactions. I mapped this in QGIS, using methods I've described previously on this blog. The tricky bits were dealing with the messy FIPS codes, dealing with the size of the dataset, and trying to decide what to label. There is quite a bit of error in the dataset (as acknowledged by the ACS people) and each individual flow line has a margin of error value associated with it, from which I also calculated the coefficient of variation. This is explained in a more detailed working paper, which I expect to publish in the coming months.

Update, 20 September 2015: there has been quite a bit of interest in the underlying dataset I put together to create the maps, so I have decided to make the whole shapefile available here in the hope that others will find it useful and be able to produce some interesting analysis or visuals from it. I'm hoping someone will do a cool interactive web map of it, but it might be quite technically challenging. If you do use it, make sure you read the associated working paper, which explains the process and the underlying data. One word of warning: the uncompressed file is pretty big so you'll need a good computer.

Mapping the American Commute: download the data (213MB, zipped shapefile)