Like night and day: How commuters change population numbers

75/85 under Ralph McGillEver wonder where all the other people stop-and-going around you during your commute are on their way to? Or how many people are in town during the week versus on the weekends? No? Well, just go with me on this.

Suburb-to-suburb commuters outnumber city-to-suburb commuters in the U.S., but in a sprawling metro area like Atlanta’s, there’s a good chance your fellow commuters are on their way not just to another town, but to another county.

For their June “regional snapshot” (PDF) the ARC used 2010 census data to find out how the morning and afternoon flow of commuters affect the population of each of the 10 core metro counties by comparing daytime populations to resident populations.

“Daytime population,” by the Census Bureau’s definition, incluldes “the number of people who are present in an area during normal business hours, including workers. This is in contrast to the ‘resident’ population present during the evening and nighttime hours.”

The tricky element of that calculation is that the estimates are based on trips made only by workers, so they don’t include people coming into an area for anything other than work, like shopping, conventions, tourism or even those on business trips.

The largest daytime change occurs in Fulton Co, where the population increases by more than 32, percent to almost 1.2 million. Clayton County’s daytime population is boosted by 12.8 percent, thanks in part to the 58,000 people who work in and around the airport. Daytime population increases in Cobb and DeKalb counties are 2.3 and 0.2 percent, respectively.

At the other end of the spectrum, Paulding County’s population decreases by 25 percent during the day, and Cherokee County’s by more than 20 percent. Barrow County’s population falls by nearly 18 percent during the day, while Henry and Coweta County’s both fall by 15.7 percent.

Although it has the second largest percentage increase in daytime population for work, Clayton County also has the higest percentage of people leaving during the day for all trips combined, work and non-work. More than 51 percent of daytime trips that originate in Clayton County end somewhere else. Rockdale County was next, with about 47 precent of all daytime trips going outside the county, then DeKalb County at nearly 44 percent.

No huge surprises here, but it’s interesting to see some numbers put to the daily migrations.

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