While the environmental damage caused by transport is the direct result of engineering design – the use of mechanical vehicles largely propelled by fossil fuels – it is human behaviour that influences the way transport is used and the infrastructure that is provided. There has been a tendency in the past to seek technological solutions to environmental problems, and there is clearly a continuing role for policies that foster this approach. But more recently, over the past 20 years or so, there has been a recognition that changing the ways transport is used, as well as the physical nature of the hardware, is at least as important. In both cases, however, there is a need to understand the responses of society to alternative policies. This entails, at one level, insights into the likely reaction of people to alternative policies – basically good forecasting models. At another level, that of project and policy evaluation, there is a need to understand the values that society puts on environmental improvement.