The transport sector forms a complex system. Some of the major driving forces act over the long term, such as decisions on land-use and infrastructure or changes in lifestyle, while other determinants such as prices can affect demand, modal split and route choice in the short and medium term. Moreover, changes in the transport system affect, and are affected by other complex socio-economic systems such as employment. Ideally, transport policy-making should take account of these inter-relations and long-term effects. However, it is often easier to use a partial model in which many variables are treated as constants or exogenous factors. Few tools can handle the complexity of real systems - one of these is known as system dynamics.