The ugly choice American cities face

From The Washington Post:

There are two ways for cities to grow when they’ve begun to run out of housing: They can expand outward, carving new neighborhoods from the countryside, or they can turn inward, replacing small buildings with larger ones, tucking new construction on unused lots, stretching higher instead of farther out.

The first option has long been the American way.

Since 1950, the vast, vast, vast majority of housing growth around U.S. cities has come from building where there were no buildings before. In every decade since then, nearly 90 percent of new housing construction in the U.S. has taken place on land that was either previously undeveloped or very low-density (with less than four homes per acre).

That’s according to a novel new analysis of American Community Survey data on the construction vintage of all U.S. housing by Issi Romem, the chief economist at the platform BuildZoom that tracks building permit and contractor data. His study uses building ages at the Census block-group level to trace where cities expanded — and what was already on the land when new housing was built.

“Densification just doesn’t account for the bulk of housing growth,” Romem says.

The above GIF of the Washington-Baltimore area shows how the two metros have expanded into each other since 1940. Nearly all of the region’s growth since then has come from the spread of new low-density neighborhoods (in light blue) into rural parts of Maryland and Virginia. Far fewer neighborhoods grew denser — darker blue — over this time.

Here is what the same timeline looks like in Atlanta:

And Phoenix:

This history — and why U.S. development has taken this shape — also suggests that it will be very difficult for cities to grow denser in the coming years, despite rising worries about the environmental costs of sprawl and the individual toll of commuting.

Romem’s data …

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