Shannon Bouton, Stefan M. Knupfer, Ivan Mihov, and Steven Swartz:

Cities move. People hurry from corner to corner; cars and trucks roll along the roads, while bicycles and scooters jostle for space.

But sometimes that movement falters, and with it the dynamism that is the hallmark of great cities. Unhealthy smog levels and traffic jams, with their chorus of horns and shouts, are routine irritations of urban lives, and things could get much worse. The world’s cities are facing an urgent set of challenges when it comes to ensuring that fundamental rite of urban living: getting around.

By 2030, 60 percent of the world’s population will live in cities, up from about 50 percent today.1 Over the same period, more than two billion people are likely to enter the middle class, with the majority of them living in cities in emerging markets, particularly China. The number of megacities with more than ten million people will continue to grow.

Many people entering the global middle class will want to buy cars: automobile sales are expected to increase from about 70 million a year in 2010 to 125 million by 2025, with more than half forecasted to be bought in cities. Some automotive analysts have gone as far as predicting that on the existing trajectory, today’s 1.2 billion strong global car fleet could double by 2030.2