A sound understanding of road user responses to changes in prices or in standards of living is crucial to making transport policy decisions. Such knowledge can inform attempts to achieve emissions reductions and help show how traffic levels might be manipulated by making some change to the cost of driving. It can help public policy-makers reach decisions about the allocation of investment and can be used to forecast how the demand for fuel and road travel will change as prices or standards of living change. It can also shed light on the implications of road-related fiscal policy for certain groups in society or certain geographical locations.