Climate change is the biggest single challenge ever faced by human civilization. Human economic activity has put so much carbon dioxide (CO2) and other greenhouse gas emissions (GHGs) into the atmosphere that serious global warming is already happening. As a society, we have no choice but to reduce these emissions drastically in order to stand a good chance of avoiding potentially catastrophic changes in our climate. Moreover, emissions from transport are rising faster than emissions from any other sector and in some cases the increase in transport emissions is counteracting emissions reductions achieved in other sectors. Lowering transport emissions presents a series of unique and formidable challenges.