We already have one billion, worldwide. Transportation researcher Dan Sperling says that stands to double within about a generation, with unthinkable consequences for air quality and climate change.

But it’s his job to think about the unthinkable. Sperling is a founder of the UC Davis Institute of Transportation Studies and a member of the California Air Resources Board. He talks about his new book, Two Billion Cars, on today’s podcast of NPR’s Fresh Air.