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A new study claims to "contradict" the premise that density discourages driving — but does it?

From an urban planning perspective, Seoul, Korea, is kind of a tale of two cities: It was the best of driving incentives, it was the worst of driving incentives.

On one hand the Seoul metro area is incredibly suburban. In 1989, the Korean government established five new suburbs around the city, each situated between 12 and 17 miles from the central business district. Since that time people and jobs have flocked outward from the city center in droves. That should mean more driving.

On the other hand it's incredibly dense. One recent estimate placed its population at 52,500 people per square mile. Meanwhile it has one of the world's most expansive transit systems, with upwards of 10 million riders a day on the subway alone, not to mention a new bus-rapid transit network. That should mean less.

The duality of Seoul — essentially a high-density suburbia — makes it a fertile test ground for smart growth principles. In the epic contest between density and suburbanization, which has a greater influence on driving behavior? A group of Korean researchers recently asked this intriguing question and, in an upcoming issue of the journal Cities, they provide an even more intriguing answer:

"… suburbanization and density are not at odds, but rather are corroborative in encouraging automobile use in the Seoul metropolitan area."

While you're busy scratching your head let's take a closer look at what's going on here. The researchers analyzed an abundance of data on metropolitan Seoul and reached several of conclusions that harmonize with conventional planning wisdom. They found that auto dependency increased with new residential developments (like the five new suburbs from 1989), for instance, and that it decreased with greater population or job density (or with mixed land-use development).

That's clear enough. But then things got a little more complicated. The researchers tried to measure the interaction between density and suburbanization that occurs when a Seoul resident chooses a transportation mode. In this calculation, instead of offsetting one another, density and suburbanization combined to increase auto usage anywhere from 17 to 27 percent depending on the year, the researchers report.

In other words, just because the suburbs in Seoul are dense doesn't make them any less friendly to automobiles, as smart growth principles would predict — in fact that made them more friendly. Their words, our emphasis:

The results of our empirical analyses provide mixed policy implications for proponents of Smart Growth. First, densification of population and employment and mixed land use may be effective Smart Growth tools for increasing transit ridership. However, our empirical evidence from the SMA suggests that suburban densification may not discourage automobile usage, which contradicts the arguments of Smart Growth advocates, who emphasize the importance of suburban densification to reduce automobile dependence.

The attack on smart growth seems unsubstantiated, says Marlon Boarnet, a planning professor at the University of Southern California who was not involved in the Cities paper. While the researchers conducted an interesting study with a strong analysis, they overreached with their conclusions, he says. What's important to remember about smart growth is that density is only one of several elements for its success. Organizing space in a way that encourages alternative transportation, particularly by placing origins near destinations, is critical too.

Take, for instance, two hypothetical development plans. In Plan A you put 1,000 residential units on 50 acres of land but don't built any places of commerce or employment. In Plan B you put 500 residential units but add in a mixture of other buildings: shops, offices, etc. If you think density alone guides smart growth, then you'd think Plan A would be less conducive to driving. But clearly Plan B accommodates more short trips by bikers, walkers, and transit riders — even though, strictly speaking, it's less dense.

What Seoul might have on its hands is a lot of Plan A's. Indeed, about 80 percent of new housing units in Seoul in 2008 were apartments; clearly there's a general demand for high-density residences in the city and suburbs alike. The authors of the Cities paper hint at a lopsided residential density when they suggest, toward the end of their paper, that Seoul should encourage more "suburban nonresidential development" to discourage more driving.

"If the authors had restated their findings as a caution against a naive view of Smart Growth which relies only on density increases, I would be much more comfortable with their conclusion," says Boarnet. "I think what the authors have found is a standard caution against focusing only on density — either only residential or employment density — and not an indictment of Smart Growth itself."

Top image: ben bryant / Shutterstock.com

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