“It’s just like every other city,” Mr. Romer said. “Except in this other way, it’s like no city ever.” White-haired and 63, he was dressed in black gear he’d bought at R.E.I., figuring black was the thing to wear at Burning Man. It was the first time that Mr. Romer, the former chief economist of the World Bank, had attended the annual bacchanal.

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Urbanization in the developed world has largely come to an end; nearly everyone who would move from farmland toward cities already has. This century, the same mass migration will run its course across the rest of the world. And if no one prepares for it — if we leave it to developers to claim one field at a time, or to migrants to make their way with no structure — it will be nearly impossible to superimpose some order later.

It will take vast expense, and sweeping acts of eminent domain, to create arterial roads, bus service, trash routes, public parks, basic connectivity.

That prospect agitates Mr. Romer, because the power of cities to lift people out of poverty dissipates when cities don’t work.1To economists, cities are labor markets. And labor markets can’t function when there are no roads leading workers out of their favelas, or when would-be inventors never meet because they live in gridlock.

Mr. Romer’s answer is to do with this moment what Burning Man does every summer: Stake out the street grid; separate public from private space; and leave room for what’s to come. Then let the free market take over. No market mechanism can ever create the road network that connects everyone. The government must do that first.

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  • 1. In Mr. Romer’s Nobel lecture, he implored people to think of cities, especially in the developing world, as places where people get the benefits of interacting with one another. A global economy built on ideas no longer has to be zero-sum, he argued. Everyone can use ideas at the same time. Someone living in America benefits if someone in India becomes better off and invents a vaccine.