It is a spring day, maybe 50 or a 100 years from now, and New York and Los Angeles are very different places from the ones you knew back in 2008.
Trucks no longer fight their way through tunnels and over bridges. There are still plenty of trucks, but they run on hydrogen and are guided in groups by computer networks.
The asphalt jungle of the 20th century is a distant memory. The city of the future, say people who work on it, will be green, both literally and figuratively.
Many older buildings remain, but they're no more than strange behemoths of brick or steel. The newer ones are different. It is most striking to see them if you fly over them in a hovercraft.
Their roofs are forests. There are trees all around them. Some homes actually use living trees as part of their structure.
Other towers are vertically arranged farms -- right there in the middle of the city -- with crops growing on sun-bleached terraces. That's part of the reason there is less traffic. Food and other supplies for the city's millions do not have to come from far away. The city is self-sufficient. Someday it may not even need streets.
This is something environmentalists say needs to happen. Otherwise, they say, the world's cities will become less and less livable.
"Greenery packed into every nook and cranny can bring some of that suburban appeal back into the urban landscape," said Majora Carter, an environmental activist from New York. "People love these things, and people love the jobs that can be created through these and other clean-tech ventures."
"By locating the production of goods closer to the point of consumption, we reduce fuel and pollution costs as well as road wear and we keep jobs here for a vibrant working class."
Carter is one of several members of a panel on "Future Cities," part of the World Science Festival in New York running from Wednesday to Sunday.
She is the founder of Sustainable South Bronx, an organization working to revitalize one of New York City's poorest areas. She argues that environmentalism is key to the effort.
"When we fix the environmental burdens that poor areas now carry, we lower public health costs, social services, law enforcement and incarceration costs," she said. "We can improve educational outcomes and hope through green-collar job creation."
Not all urban ills are environmental, of course, and not all solutions will produce the utopian visions of city life that used to be a staple of science-fiction movies. But these are major issues. A U.N. report says half the world's population will live in cities by the end of this year, and the number will grow to 70 percent by mid-century.
In many ways, urban planners say that's a good thing. When people are close together -- and feel comfortable that way -- they have a greater sense of community. They have greater access to things they need, and to cultural amenities.
And cities are energy-efficient. The Census Bureau reports that almost half the commuters in New York City use mass transit, compared to 2 percent in sprawling suburban areas such as Orange County, Calif.
"The transportation factor is a major consideration," said Mitchell Joachim, an architect and urban designer. "U.S. transportation uses 29 percent of our total energy needs, and this figure is expected to double in 40 years."
Joachim says that over time, cities naturally reshape themselves. But today's cities are dominated by steel and asphalt, and that, he says, has to change.
"We are tired of cities that force people to move around in rigid, clanking, cumbersome, often dangerous metal capsules -- cars, trains, elevators, escalators and all the rest," he writes. "We propose cities that are softer, gentler and more sensual."