Traffic Demand Management

Traffic demand management (TDM), otherwise known as transport demand management or mobility management (especially in Europe), is a term that refers to a range of actions (for example, policies, programs, and projects) that modify the demand [Page 1354]for transport infrastructure and services in either space or time in order to improve the efficiency of the transport system. The term is distinguished by actions that respond to increases in the demand for transport by adding to transport infrastructure or capacity (that is, supply management), such as extending roads, expanding airports, or increasing space for parking facilities.

The origins of TDM can be traced back to the 1970s when the oil crises of 1973 and 1979 prompted various attempts in the United States to increase the fuel efficiency of transport and reduce oil dependence. TDM was also seen as a way of addressing other economic and environmental policy issues of the time, such as reducing traffic congestion (and thereby helping increase economic productivity) and improving air quality, which had become increasingly important in the United States since the 1960s.

In the 1980s, TDM gained prominence because it was seen as a cheaper and more effective way of dealing with increases in motorization than building additional transport infrastructure. The expansion of existing transport infrastructure, especially in urban areas, was facing increasing difficulties due to the high price and limited availability of land for new infrastructure, the financial constraints of public authorities responsible for providing the infrastructure, and the additional “external costs” on society and the environment associated with the provision of new infrastructure (for example, noise, health, air pollution, and severed communities).

Interest in TDM continued to grow in the 1990s, particularly in Europe, when the underlying philosophy of transport and infrastructure policy experienced shifts in many countries, moving from a predominantly supply-based “predict and provide” approach toward a new realism in which there was greater recognition that, even if new infrastructure is built to accommodate the expected increases in transport demand, levels of traffic, congestion, and pollution can still worsen because there is a substantial latent demand for transport.

TDM encompasses a wide range of measures to influence whether, how, when, where, and how often people travel. TDM measures are mostly related to urban passenger travel rather than intercity travel or freight transport, although the concept is also applicable to these policy areas. Although building additional transport infrastructure or capacity can technically be categorized as supply management rather than demand management, increasing the capacity of public transport infrastructure or infrastructure for “green” modes (for example, walking and cycling) is generally considered to be a form of TDM.