They proposed no grand new time-consuming, budget-breaking monuments, but a suite of modest new bridges, along with the renovation of some great historic ones, amid a variety of green spaces. The park was to be generally informal, low-key and practical, in certain respects more American than European, full of playgrounds and ball fields and bike paths.

Most important, it would be constructed in stages. Every month another section could be rolled out. The mayor wanted to stand with grateful citizens in front of news cameras in the first section of the park before re-election day in 2007. He did. Public grumbling about traffic jams gradually morphed into praise for a new green space.

Of course Madrid is now just about broke, and Mr. Gallardón’s opponents point to his civic improvements as one cause. They were indeed expensive, albeit a fraction of what the costs would have been in America. Pilar Martínez, who oversaw the park project in the mayor’s office, told me that the official price tag of Madrid Río hovers near $5 billion, all but $500 million of it spent to bury the highway. Twenty-seven miles of new tunnels were dug; countless tons of granite installed to make paths and fountains; some 8,000 pine trees planted. A new, elegantly simple boathouse has been designed, and a 19th-century complex of brick and glass buildings, including a derelict slaughterhouse and greenhouse, are now being renovated to house art studios and a dance theater.

Add to this wading pools for toddlers that landlocked Madrid parents already fondly call “the beach,” and a paved plaza, in patterned tiles, large enough to fit a few hundred thousand people.

New York has recently benefited from the growth and upgrading of its own parks, but much of the city’s expanding public realm is now dependent on private investment. At the epicenter of laissez-faire capitalism, a skepticism about big government, a web of well-meaning regulations and opposition groups empowered by easy access to the courts combine to create barriers to the investment of public money in major infrastructural improvements. Change happens slowly and incrementally, certainly compared with what Madrid has accomplished.

The sort of visionary will that produced W.P.A. projects from which the country continues to benefit seems almost anachronistic. It takes the rare political strongman, like Mayor Richard M. Daley of Chicago, to push through something big and great that is entirely for the public, like Millennium Park, 24.5 downtown acres of cultural attractions risen largely from rail yards and parking lots. (And the State of Illinois is now contemplating a 140,000-acre park, potentially the largest urban park in America, on underused and post-industrial land on Chicago’s southern edge, but for the moment it’s just an idea.)