Mobility is rapidly becoming one of the greatest challenges facing developed and developing countries alike. Vehicles account for 20 per cent of greenhouse gas emissions. There are also rising concerns about their impact on the quality of urban life, including social inequities, and about the effects of their pollution on health and buildings. Billions of hours are being lost in congestion, with commensurate financial losses for economies businesses and individuals. According to the Texas Transportation Institute, for example, congestion in United States in 2007 cost close to $90 billion a year, due partly to four billion hours — and 11 billion litres of fuel — lost in travel delays.