The key to solving loneliness and pollution might be giving up cars and hanging out in the street. It's working in Barcelona.

• Barcelona is a decade into transforming many of its streets into green, car-light public spaces.
• The effort is admired by urban planners around the world and could be a model for US cities.
• Americans are increasingly demanding dense, walkable, mixed-use neighborhoods.

Barcelona may have found an answer to a whole host of social, health, and environmental ills.

The city began implementing "superblocks" — "superillas" in Catalan — a decade ago as a way to cut air pollution. The project essentially limited through-traffic in a cluster group of nine square city blocks, creating greener and more pedestrian-friendly streets.