photo: a cul-de-sac in Utah.
We're on a road to nowhere. George Frey/Bloomberg

Satellite images dating back to 1975 allow researchers to map how millions of cul-de-sacs and dead-ends have proliferated in street networks worldwide.

A connected street is a healthier street. Neighborhoods with more short links and intersections—and fewer dead-ends and cul-de-sacs—have lower rates of obesity, diabetes, and heart disease, researchers have found. In part, that’s because they promote walking and biking.

So what does it mean that the world is growing less connected?

A new study published in the Proceedings of the National Academy of Sciences charts a worrying global shift towards more-sprawling and less-hooked-up street networks over time.

Adam Millard-Ball, a UC Santa Cruz professor of environmental studies, and Christopher Barrington-Leigh, a McGill University economist, examined 28.6 million miles of streets across every continent, using public data from OpenStreetMap as well as historic satellite imagery. They built an algorithm to identify various characteristics of connectivity, such as the number of cul-de-sacs, the length of unbroken street links, and how long it takes to walk to key destinations.

(File this under “the more you know”: There are 10,845,867 dead-ends in the world, at least mapped on OpenStreetMap.)

Combining this information to create an index for street “disconnectivity,” Millard-Ball and Barrington-Leigh then mapped it across the globe. In their interactive online Global Sprawl Map, the bluer the area, the more compact its streets tend to be. The redder, the more sprawling.

The researchers’ street disconnectivity index, mapped since 1975, shows a trend toward global sprawl. (PNAS)

Most of the patterns are what you might expect. Cities in Latin America, Japan, South Korea, much of Europe, and North Africa tend to feature more tightly connected streets, while sprawling urban and suburban areas in Southeast Asia, the United States, and the U.K. have looser grids and more cut-off segments. And because the researchers were also able to map changes over time through satellite imagery, they also found a global trend since 1975 towards disconnectivity. Right now, sprawl has migrated from American suburbs and is proliferating quickly in Southeast Asia, India, and other parts of the developing world.

A tool within the map allows users to zoom in on towns and cities with various kinds of street networks, and trace how these networks have grown since 1975. Here’s an example of a largely connected grid: downtown Palo Alto, California, which has only a handful of dead-ending streets (in red).

Downtown Palo Alto, California, has high street connectivity. (Screenshot: Global Sprawl Map)

And here’s a newer suburb outside Atlanta, which abounds with disconnection. The green routes indicate only-way-in streets.

Suburban Atlanta: Cul-de-sacs galore. (Screenshot: Global Sprawl Map)

Who cares? Apart from the fact that spread-out street networks tend to be less healthy for the people who live on them, they also create a greater burden on society, the researchers argue. Developers building new spread-out neighborhoods in fast-growing cities don’t account for the costs—in time, fuel, and greenhouse gas emissions—associated with such car-centric patterns. But those costs exist, and as the map shows, building streets that require them creates a pattern that persists over time. That, in turn, erects barriers that prevent cities from encouraging their citizens to adopt more sustainable ways of getting around, and living.

“If you build a disconnected neighborhood, you’re transit-proofing that neighborhood for the next century,” said Millard-Ball. “If you build a single-family home on a tightly gridded street, a few decades later it’s not hard to change that into a duplex or high-density housing. But if you start with the wrong street, they’re really hard to fix.”

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