Transportation demand management (TDM) is an umbrella term used to describe a variety of actions aimed at reducing or modifying the demand for transportation facilities and services. The actions include congestion pricing/value pricing; electronic road pricing; ridesharing/car pooling; improvements to public transport; and parking facilities. The term derives from a contradistinction with actions that respond to congestion by increasing the supply of transportation facilities or capacity (i.e., supply management), for example, the expansion of highways or airports to accommodate peak traffic. TDM does include some supply measures, however, when they focus on alternatives to the mode in question. Hence, increasing the supply of transit or other non-highway capacity is generally seen as a form of TDM, and in some cases expanding intercity rail capacity is seen as a form of TDM for airports. TDM has gained attention since the 1970s primarily as a result of significant increases in travel that have not been accompanied by increases in infrastructure capacity. Demand for transportation facilities and services typically grows out of economic expansion and prosperity. However, commensurate expansions in infrastructure capacity have become more difficult. The cost of infrastructure expansions is often borne by public authorities that are unable or unwilling to commit the requisite financial resources. The construction of new facilities or the expansion of existing facilities also often brings with it "externalities" such as noise, air pollution, and incursions into open space or already built-up communities. TDM has become increasingly appealing as supply responses to congestion have become more difficult. Support for TDM also has a normative component, insofar as some groups believe that increasing mobility, at least in some modes of transport, is incompatible with environmental sustainability.
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