When it comes to transit, there are two types of American cities. The first are the six “legacy” cities -- Boston, Chicago, New York, Philadelphia, San Francisco and Washington, D.C. -- that developed dense downtowns long before the automobile and have always maintained significant transit ridership. Then there’s everyone else.

Despite recent urbanization, most U.S. cities were designed for cars. In fact, most remain dominated by solo drivers. Taking transit in these cities is generally inefficient. Nevertheless, some, such as Los Angeles and Miami, are arriving at an “in-between” stage. They’re seeing increased density and congestion. These kinds of cities face two major questions: Can they densify in ways that avoid traffic Armageddon, such as by encouraging transit? And can transit ever become as convenient as driving once was, so that densification doesn’t mean reduced quality of life?

This second question, in particular, matters. As I’ve discovered while traveling across the country, America’s densest, most transit-intensive cities are also the hardest places to get around, precisely because people can’t quickly drive to and park at their destinations. It helps explain why the legacy cities have six of America’s 11 longest average commute times. It’s therefore crucial that in-between cities densify in a way that preserves personal mobility.

That is where the in-between city of Seattle could become an interesting case study. As America’s fastest-growing big city -- it added 21,000 people in 2015 -- it’s growing denser and more congested. Fortunately, it already has a robust transit system: 70 percent of daily commuters living downtown use transit, and Seattle’s countywide bus ridership since 2002 has grown at a faster rate, more than double, compared to the population.

This growth has occurred because residents in Seattle adequately fund transit. In two decades, voters have approved three pro-transit ballot measures. As a result, the city pays more per capita on new transit projects than any other metro area. Recently, it has invested this money into its buses by expanding access so that 25 percent of residents live near frequent service; designating transit lanes to keep buses from getting stuck in traffic; and building an underground tunnel, which opened in 1990, so that buses can avoid downtown traffic.

Beyond funding, though, Seattle has -- at least relative to other U.S. cities -- encouraged approaches that complement transit. The private rideshare industry, for example, has flourished there, accounting for 9 percent of downtown commutes. The city has also relaxed zoning laws to allow for denser development.

Time will tell whether these measures help Seattle maintain, or even improve, its mobility. Already, average congestion and commute times there remain below that of legacy cities. The real test will be keeping it that way even as it continues to get denser and bigger.