IVETTE FELICIANO:

Fifty percent of the world's population live in urban areas, but that will grow to 70 percent by the year 2050, according to the United Nations. Today, there are 31 mega-cities..metropolitan areas with more than 10 million people: Tokyo, Delhi, Shanghai, Mexico City, Sao Paulo, Cairo By 2030, the UN predicts, there will be 41.

As the number of city dwellers rises, so do problems like overcrowding, pollution, housing shortages, and aging infrastructure like mass transit and highways