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The Cul-de-Sac Nightmare

Human health and the environment have suffered from urban planning trends dating to the 1920s, but that trend is starting to reverse.
(Photo: Michael Patrick/Flickr)

(Photo: Michael Patrick/Flickr)

Hear the phrase "urban sprawl," and there's a decent chance you think of Los Angeles, that never-ending mass of freeways and streets that seems to extend from the Pacific halfway to Twentynine Palms. But in fact, the worst offenders may be found in lesser-known places like Redmond, Washington. It was there, in the 1990s, that a trend toward cul-de-sac-laden suburbs reached its nadir, bringing with it accompanying consequences for health and the environment. Fortunately, that trend may finally be reversing, according to a new study.

To understand why Redmond and its ilk take the sprawl-king crown, here's a quick lesson in planning streets: Generally speaking, the more intersections there are and the more streets meeting at an intersection, the slower traffic will be—and that can be a good thing. Downtown streets, for example, are chock full of four-way intersections that slow down traffic enough that many people would rather walk or take the bus than drive.

It's in this respect that Redmond has a problem. Rather than a grid or tangled mess, its streets are like the branches of a tree, terminating in a sea of cul-de-sacs. That's great for driving but bad for your health and, because more driving means spewing more carbon into the air, bad for the environment. But when did that disaster of a planning concept begin, and how has the average intersection changed over time?

In the 1990s, a trend toward cul-de-sac-laden suburbs reached its nadir, bringing with it accompanying consequences for health and the environment.

To answer that question, researchers Christopher Barrington-Leigh and Adam Millard-Ball constructed a database of intersections throughout the United States including not just their current incarnation, but also how they've changed over roughly the last century. To measure street connectivity, the pair used the concept of node degree, a term borrowed from network science that quantifies the number of paths in and and out of a point—in this case, the number of streets going in and out of an intersection. A four-way intersection has degree four, for example, while a T-intersection has degree three.

Barrington-Leigh and Millard-Ball found that the average node degree of America's streets hit a low of about 2.6 in the mid '90s, but that trend dates back to the design of Radburn, New Jersey, and similar, cul-de-sac-heavy garden city layouts. In 1920, the average intersection node degree was about 3.2, reflecting a mix of intersection types. That dropped slowly until the post-World War II period, when growing car ownership seems to have accelerated the decline that culminated in, well, Redmond.

But then came a reversal. By 2012, the average node degree had gone up to 2.83, possibly because of new municipal policies encouraging denser neighborhoods with more interconnected street systems.

Such policies "can be seen as just one element in a package of policies to promote denser, mixed-use, connected development patterns," Barrington-Leigh and Millard-Ball argue. "Pursuit of this agenda can shape the fundamental infrastructure and incentives that guide future sustainable development" in the U.S. and abroad, they write.

Quick Studies is an award-winning series that sheds light on new research and discoveries that change the way we look at the world.