Cities are often places of great energy and optimism. They are where most of us choose to live work and interact with others. As a result, cities are where innovation happens, where ideas are formed from which economic growth largely stems. Although wrestling with difficult challenges, sometimes individually but more often collectively, cities are able to address some of the most significant challenges facing society today. City councils around the world are reducing air pollution, banning diesel cars, introducing smoking bans, bicycle rental schemes and even imposing sugar taxes in order to...