Do you know your city's "commuter-adjusted population"?

About 1.5 million people live in Manhattan, an imposing number that's larger than the entire populations of Phoenix, Dallas and San Francisco. More impressive, though, is what happens on the island by day: So many commuters come in (and so few residents commute out for work) that Manhattan's population nearly doubles in size.

This latter number – 3,083,102, to be precise, according to American Community Survey data collected between 2006 and 2010 – is in some ways an even more important one than the population figure we typically affix to places. If Manhattan ever needs to evacuate by day during a disaster, the city has to figure out what to do with all 3 million of those people. The city's transportation planners are responsible for every one of them, whether they live in New York or not. And anyone who does business in a service industry on the island – from lunch counters to dry cleaners to department stores – cares a lot more about how many people pass through during the day than who passes out in Manhattan at night.

"Commuter-adjusted populations" tell us a lot about where the jobs are and which communities do little more than give people a place to sleep. Count people where they work, and not where they live, and the resulting picture also further blurs the divide between cities and suburbs (and how we think about who is invested in which places).

As a major jobs center, Manhattan not surprisingly has one of the most dramatic changes in daytime population. Percentage-wise, though, the growth of Redmond City, Washington, is even greater; the population of the Seattle suburb expands by about 111 percent by day. Why? Microsoft is based there.

These figures come from a new report from the Census Bureau using data from the American Community Survey, which asks people both where they live and work. These cities are among the top 20 for commuter adjusted population (here we're looking at all of New York, not just Manhattan):

Census Bureau

Invariably, those cities that lose the most population by day tend to be outer-ring suburbs with lots of homes and few jobs (Centerville and Dale City, Virginia, outside of D.C.; Atascocita, Texas, north of Houston). This map contrasts those patterns of daytime population loss and gain across the country, with big receiver counties in blue and feeders in red.

"Commuter-Adjusted Population Estimates: ACS 2006-10"

In the Northeast, there are a lot of counties with heavy daytime population loss immediately adjacent to New York, Baltimore, Philadelphia and Washington, making those cities commuting hotspots. The differences are less stark out West, where population densities are lower and there are fewer centralized employment clusters.

This same data can be converted into a ratio between the number of workers living in a given area to the number of jobs there. Manhattan clearly has more jobs than its own worker population can handle, by a ratio of 2.81 to one. That means, in short, that it imports a large amount of its labor. Washington, D.C., does the same, with 2.58 jobs for every worker who actually lives in the city. Counties with a ratio below one essentially export workers – they are bedroom communities more than employment hubs.

"Commuter-Adjusted Population Estimates: ACS 2006-10"

This geography of how populations move on a daily basis should also tell us something about the importance of regional transportation infrastructure. If your city swells in size every day by 50,000 people or more, do you want all of them coming by car?

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