As Charles Montgomery reveals, it's not how much money your neighbours earn, or how pleasant the climate is that makes the most difference. Journeying to dozens of cities - from Atlanta to Bogotá to Vancouver - he talks to the new champions of the happy city to explore the urban innovations already transforming people's lives. He meets the visionary Colombian mayor who turned some of the world's most dangerous roads into an urban cycling haven; the Danish architect who brought the lessons of medieval Tuscan towns to modern-day Copenhagen; and the New York City transport commissioner who turned the gridlock of Times Square into a place to lounge in the sun.