Two Guangzhou-Shenzhen-Hong Kong Express Rail Link trains in Hong Kong. Wider use of fast trains like these, or the projected Hyperloop, could make it practical to live and work farther apart.

The hyperloop, Elon Musk has boasted, could whisk you from New York to Washington in 29 minutes. Other maglev boosters sell similar dreams: San Francisco to Los Angeles in under 30. Dallas to Houston, Portland to Seattle, Orlando to Miami in the same.

The half-hour trip is something of a mystical notion in transportation. These visions of the future sound seductive in part because half an hour is, in fact, roughly how long many of us spend getting to work. The typical American commutes 26.4 minutes, one way, according to the American Community Survey. Even in metro New York, with nearly the longest commutes in the country, that average is 36 minutes.

Of course plenty of workers trek less or much more, but average American commute times have budged only modestly over the last 35 years, since the census began asking about them. International studies have shown similar half-hour patterns. History even hints that the Romans traveled about the same, when most people went everywhere on foot.

Ancient Rome, here depicted by a 17th-century artist, was about five kilometers across, walkable in an hour or so.

The curious stability of the half-hour average commute means that when bullet trains — or autonomous vehicles, or whatever innovation comes next — link two places by that much time, they won’t just open up plausible new weekend getaways and airline alternatives. They will also potentially restructure daily life: where people live, what jobs they hold, how cities expand over time.

“What Musk correctly realizes is that there will be a huge market with maglev or hyperloop technology for the places it connects in 30 minutes,” said Jesse Ausubel, an environmental scientist at the Rockefeller University. “Any pairing that you can fit into that more or less one-hour round trip, the traffic will multiply immensely,” he said, referring to the volume of travelers.

People priced out of Brooklyn could move to Baltimore. Congressional aides would commute to Philadelphia. Whole cities — and labor and housing markets — would fuse together.

The hyperloop is a wild hypothetical. But Mr. Ausubel’s point stands on two related patterns from history. When you give people greater speed, they don’t use it to save time; they use it to consume more space. As a result, cities have spread outward as transportation technology has evolved. Horse-drawn carriages enlarged pedestrian towns. Streetcars enabled streetcar suburbs. Highways made exurbia possible.

What, then, will cities look like with true high-speed rail, or autonomous cars, or even the hyperloop? What happens when 30 minutes of time buys you not two miles, or 10, but 200?

A railway platform in Hoboken, N.J. Investment in infrastructure could transform American commutes.

Transportation futurists partly anticipate this question (if not all the ripple effects their innovations will bring): “We’re not selling transportation,” the company Hyperloop One says. “We’re selling time.”

And time, in transportation, means territory.

The accompanying maps illustrate how that territory changes as the means of travel does. Starting at the 30th Street Station in Philadelphia, a city once crossable on foot, the transportation analysis consultancy Conveyal plotted for…