National Public Radio: Transportation expert Daniel Sperling estimates that the world’s car population – which currently stands at one billion vehicles – is likely to double in the next 20 years. Sperling is the co-author (with Deborah Gordon) of Two Billion Cars: Driving Toward Sustainability, a book that considers the environmental impact of so many automobiles and suggests ways that politicians, car companies and the general public can curb car-ownership and reduce climate change. Sperling is a …