I was asked recently how the center of the population of the state may have changed between 2000 and 2010. The answer is it didn’t! Let me explain.
There are two different measures of population centrality. The traditional one, used by the Census over the years, is called the bivariate median or center of gravity. It is actually a lousy measure because it minimizes the square of the weighted distances, giving excess influence to the most remote corners, but it was easy to calculate. The better measure is called the bivariate median, or point of minimum aggregate travel, much harder to calculate.
The traditional center of gravity of the state’s population has moved gradually westward from 1900 to 2010, starting near Salmon La Sac in Kittitas County, moving just west of Snoqualmie Pass by 1920, back a little bit east of the pass in 1970, but back westward to just south of the Bandera airstrip, south of I-90 by 1980 where it remains (no people around!).
Conversely, the minimum travel point is, voila! right here in Pugetopolis, and it has been continually from 1900. Using detailed geography of census tracts for 2000 and 2010 there is no change: the minimum travel center is right in the city of Tukwila, just northwest of Southcenter. Were these guys smart, or what?
So here's a more politically salient question: How will population change affect legislative districts? Imbalances of population show 21 districts with a surplus population, 28 with a deficit. The biggest surpluses are 20 to 24,000, the biggest shortfalls 15 to 18,000 — none drastic enough to require any massive realignment. Both sides of the state grew at about the same rate, so the main story should be a familiar one: relative losses in the core urban districts from Everett, through Seattle to Tacoma, with gains in the suburban and exurban districts in King, Snohomish, and Pierce counties.
In eastern Washington the substantial growth around the Tri-Cities (8th and 16th legislative districts) and the moderate loss in Spokane’s 3rd will require some adjustment, as to the 9th, but nothing drastic.
In western Washington, the story is that districts in the southwest and west have a small aggregate surplus of 23,000, mainly because of fast suburban growth in Clark (Vancouver). In the north Sound, there is a similar moderate surplus of 29,000, so the slightly underpopulated districts of the Puget Sound metropolitan core will have to nudge out a little. The 14 urban core districts from Everett to Lakewood, have a collective shortage of 141,000, while the suburban and exurban districts have a net surplus of 92,000. The most overpopulated districts are the 2nd (southern Pierce), the 5th (east King) and the 44th (Snohomish), but the population changes are not drastic enough to require any big realignment. But of course the devil will be in the details of needed adjustments.
The imbalances do certainly reinforce the observation that population change in the 2000-2010 period did not reflect urban planning goals of concentration, even in King County, but rather the continuing saga of suburban growth, across the entire state. If the reader asks how this could be — given the large amount of urban construction, and the woes of the housing sector — the answer is the basic divergence of the preferences and behavior of families and non-families.