The combustion engine is dominant. In the United States, according to the latest estimates from the Census, more than 76 percent of us get to work alone in a car. The numbers are not quite as lopsided in some big cities, where public transit and other options are more widely available.

But in urban planning circles, many people look at growing urban populations and the congestion on city streets with concern. What will be the transportation mode of the future?

University of Rochester astrophysicist and NPR commentator and blogger, Adam Frank, thinks about cities from a planetary point of view. He's interested in the effect that our urban environments have on the natural one. He's thinking about climate change. He tells NPR's Kelly McEvers to have a more environmentally friendly form of transportation, we have to create new infrastructure first.

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