But fresh thinking has focused on cheap, quick, temporary and D.I.Y.-style approaches to creating public space — among these, curbside “parklets” in San Francisco and a communal farm on what had been a derelict parcel in the middle of Phoenix. “Small steps, big changes,” as Janette Sadik-Khan, the New York City Department of Transportation commissioner, described the logic of plazas like that at New Lots.

And guess what? A beer garden made out of freight containers on an empty plot turns out to be a lot more popular and better for a city than a sad corporate atrium with a few cafe tables and a long list of don’ts on the wall.

As more and more educated Americans, especially younger ones, are looking to move downtown, seeking alternatives to suburbs and cars, they’re reframing the demand for public space. They want elbow room and creative sites, cooked up by the community or, like the plaza program, developed from a democratic mix of top-down and bottom-up governance.

The other day I visited Michael Bierut, whose design firm, Pentagram, has drawn the maps that accompany the new bike-share program. Pentagram’s New York office faces Madison Square Park. Mr. Bierut remembered when the plaza program started to take over the pedestrian-unfriendly territory where Broadway crosses Fifth Avenue, just next to the park. Traffic patterns improved, but he still thought the city was nuts to create plazas from concrete islands marooned between busy boulevards when there was already, right there, one of the most gorgeous parks in the city.

“Was I wrong,” he said, laughing.

The plazas outside his building are mobbed on warm days, with people even toting Shake Shack burgers out of the park to sit next to all the traffic — partly for the view (the Flatiron building one way, the Empire State Building the other) but also for the reason people gravitate to Trafalgar Square in London or the Piazza del Campo in Siena, Italy.