There is a very clear fault line in America's urban development pattern. A handful of so-called "legacy cities" grew before the automobile, first thrived on walking and transit, and built housing units that were dense and attached. These legacy cities have preserved this character, marked by high Walk Scores, density levels and transit ride shares, and by their urban feel. Included are New York, Boston, Philadelphia, DC, Chicago and San Francisco (and perhaps New Orleans and Baltimore, although those cities have hollowed out through the decades).

Then there is pretty much everyone else. Most U.S. cities were built following World War-II, function around the automobile, and do not have a contiguous urban fabric. Atlanta, perhaps the quintessential American city, is an example; it has a reasonably vibrant downtown and some cool surrounding neighborhoods that are tough to walk to, but is still dominated by single-family sprawl housing.

Several cities, however, are transitioning into legacy status, including Los Angeles, Miami and Denver. But the main one, when accounting for housing density and transit use, is Seattle.

Housing

The main cause of Seattle's urban evolution is, first and foremost, that people want to live there. The city has had a strong economy since the 1980s, with the rise of Microsoft. Now Amazon, which has more office space than Seattle's next 40 biggest employers combined, has made the city the modern embodiment of a company town.

More than other West Coast tech hubs, though, Seattle builds to accommodate this inward flow of jobs and employees. According to Census data, the Seattle metro area has had, since 2010, roughly double the housing construction as San Francisco-Oakland-Hayward. Further data from The Market Urbanism Report shows this discrepancy dating back to 1990.

Much of this housing growth is emerging via dense, multi-family, centrally-located units -- a.k.a in legacy city fashion. Data from the U.S. Department of Housing and Urban Development finds that, since 2000, the rate of single-family home permits in metro Seattle has declined, while the rate of multi-family home permits has skyrocketed. In 2017, Seattle is set to build over double the number of multi-family units as it has in any previous year. And it is all going up fastest in the neighborhoods in and around downtown.

Further analysis from the Seattle Times finds that the city is the nation's 10th-densest, trailing Miami, Los Angeles, Long Beach, and the 6 legacy cities. And it is the nation's fastest growing and densifying city, adding nearly 20,000 in city population per year since 2010.

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The fruits of this growth become evident when walking around the central areas here. Downtown, with a residential population of 70,000, is practically a city unto itself, and is abutted by South Lake Union, the nation's most populated work campus. Capitol Hill, just up the hill from downtown, is a moderate-density neighborhood of townhomes and mid-rises. Other neighborhoods, including Queen Anne, Fremont, Ballard, Squire Park and The U District, give Seattle that contiguous, or near-contiguous, urban feel.

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Transit

So if Seattle has around the same density as Miami and Los Angeles, why am I not writing fawning articles about how they, too, are becoming legacy cities? The big differentiation is transit. While the latter two have had trouble growing a viable network, Seattle has become America's leading transit success story. Since 2002, bus ridership rates have doubled population growth rates, as the city has expanded its frequent bus network to reach 25% of residents. Among the 50 largest cities, Seattle has, at 18%, the 7th-highest commuter transit share usage—trailing only the 6 legacy cities. In the downtown area, 70% of daily commuters arrive by alternative means, and this figure is growing.

Much of the advances come from a service, King County Metro Transit, that seems unusually competent for a public agency, and in a nation where transit ridership is declining. I found that while living in Seattle, buses were faster and more efficient than in most American cities, whisking me from my West Seattle home to downtown and other areas citywide.

Private transit also does well here. Ridesharing accounts for 9% of downtown commuter trips, and Seattle has even become the epicenter for America's private bikeshare industry. Following a failed city-run program, several companies have swooped in to provide cheap monthly memberships, most notably Limebike. The reason, of course, that any transit service can function is that a critical mass of money and people has concentrated in Seattle.

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That doesn't mean that Seattle has taken a unified march towards density--nor is immune from Nimbyism. The ongoing growth debate, says Dan Bertolet, an analyst for a local urban affairs think tank called the Sightline Institute, is about what to do with single-family areas. He says that Seattle's "urban village" land-use concept—which allows mid-rise housing along existing retail corridors by neighborhood—was employed in the 1990s as a compromise. It would keep housing growth out of the rural areas (in accordance with Washington state's Growth Management Act) and out of Seattle's residential neighborhoods.

The problem is that there hasn't been enough housing in areas slated for development to stabilize prices: median home prices in the city proper continue to grow rapidly, now up to $690,000. Nor does the legacy city look extend to most parts of Seattle.

“Look up a map of Seattle’s urban villages, you’ll see these little nodes surrounded by a sea of yellow,” said Bertolet, referring to the color of the single-family-residential zoning designation.

This might be why, for all its urbanization, Seattle is not already a true legacy city. Anti-development regulations still swamp every last parcel--including in the densest areas--preventing Seattle from yet having, say, the skyline of Chicago or the rowhouse character of San Francisco. Although Bertolet was unwilling to speculate too confidently, he said that loosening these regulations would at least bring change. Capitol Hill and South Lake Union, just a stone's throw from the central business district, would likely see new taller towers; while so-called missing-middle housing like duplexes would fill in the single-family neighborhoods.

In other words, without government imposition, Seattle would grow more urban. But the strides it is already making to become the next legacy city testify to its economic might, and to America's growing urbanization.

[This article was originally published by Forbes.]