Transportation defines our civilization. Where we live and work, the structure of our cities, the flow of global commerce--all have been shaped by transportation technologies. But modern transportation's reliance on fossil fuels cannot be sustained. Passenger planes, trains, and automobiles were responsible for nearly four billion tons of carbon dioxide emissions in 2005--about 14 percent of the carbon dioxide emitted globally that year. If we continue to rely almost exclusively on petroleum to power these vehicles, they will be responsible for 11 billion to 18 billion tons of carbon dioxide emissions in 2050. That's because developing nations--which are home to 82 percent of the world's population and will be responsible for 98 percent of population growth in coming years--are on the verge of mass motorization.