By 2050, the world's population is expected to reach 9.8 billion people. According to a report by the United Nations, nearly 70% of them are projected to live in urban areas. If current patterns hold, those who flock to the megacities of the future will face issues with affordable housing and increased hours in traffic. The impact on low-income pepole and the environment will be especially acute. National Geographic 's special edition issue on the future of cities explores how planners,