"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."