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JUST IMAGINE: A fanciful vision of New York City.

Back in April, as IEEE Spectrum’s editors were putting the final touches on this issue, Mayor Michael Bloomberg of New York City announced a sweeping urban development program, one of the city’s most ambitious ever. Such a high-profile press conference, decades ago, would have probably trumpeted a stupendous new bridge, a tunnel, or a mass-transit system. But on that warm Sunday afternoon, Bloomberg proposed a dizzying assortment of 127 different initiatives and regulations aimed at cutting the city’s greenhouse-gas emissions 30 per­cent by the year 2030.

Audacious as it was in scope, the plan didn’t really have anything new. Charging motorists to come into the city center during the day? London’s been doing it since 2003. Planting a million new trees? Adelaide, Australia, started doing that in 2002. Replacing dirty, decrepit, and ancient electric generators with shiny, new, and efficient ones? Since when did that count as a masterstroke?

Indeed, many of the technologies Bloomberg wants deployed in New York, one of the five megacities featured in this issue, will be brought together for the first time in Dongtan, a planned minimetropolis being constructed within the megacity of Shanghai [see “How to Build a Green City,” in this issue].

Still, Bloomberg’s proposal for New York, with its fractious politics, bureaucratic inertia, and byzantine land-use deals, was an unmistakable sign that something big was happening. The sustainable-city movement had reached second gear.

And yet, as necessary as all this is, you can’t help wondering how it would have disappointed the visionaries who, 75 years ago, figured that our cities would be way beyond all that by now. They saw the city of the future as a vast playground of gleaming towers connected by whooshing people-mover thingies. A place where great thinkers advanced civilization in climate-controlled comfort, while wearing utilitarian but smartly designed uniforms.

That vision reached some sort of pinnacle in the 1930 motion picture Just Imagine, an extraordinarily bizarre art-deco sci-fi musical. For the picture, Fox Film Corp. constructed, in an airship hangar, a fantastically detailed model of New York City in 1980 [see photo, “Just Imagine”]. Absurd though the movie’s plot is (pills have replaced food, and babies come from vending machines), this model city was supposedly constructed under the guidance of engineering and urban-planning experts. It took 205 engineers and craftsmen five months to build the model, and it cost about a quarter ­million dollars, at a time when that kind of money could buy an oceanfront estate.

The model resembles the urban visions of two classic films of that same era—Metropolis and Things to Come. You simply can’t imagine the leaders of these futuristic cities agonizing about air quality, traffic patterns, and the condition of the school-bus fleet. No, in Metropolis and Things to Come, they have bigger things to contend with: a subterranean slave-labor problem and a Luddite insurrection, respectively.

Technology isn’t going to bring us anything like these visions, with their whooshing people movers, spaceports, and robot servants. What it will bring, as the articles in this issue describe and as Bloomberg’s speech confirms, is greater sustainability. We’ll get much more efficient energy use, transportation, and waste disposal. Technology will also deliver greater security, as computers and sensors are deployed to correlate and disperse data about crime and terrorism, to warn of big earthquakes, and to monitor and plan mass-transit operations.

In developing cities such as Mumbai, it’s starting to become clear that technology will also be increasingly used to help isolate a privileged minority from the instabilities and deprivations of life in the third world. (Hey, shades of Metropolis—now there’s some irony!)

And we’ll always have those old, quaint, luminous, utopian archetypes, if only on our television screens. Someday they’ll remind us that after we make cities sustainable, there will still be those ridiculously tall towers and maglev people movers to get started on.