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Departments of Transportation (DOT), Metropolitan Planning Organizations (MPO), local governments and transportation providers rely on travel forecasts to support transportation plans and to prioritize investment policies. The basic premise of travel forecasting is to answer questions about future transportation system performance. Travel forecasts are used to understand the effects of population and economic changes as well as proposed transportation investments and policies. These changes, investments and policies are typically explored in the form of different future regional scenarios.
A scenario might be a simple extrapolation of existing conditions, or it might imagine more significant changes. Examples of significant changes might include user attitudes towards car ownership, new employment centers, new types of vehicles, and so on.
A scenario includes a description of the assumed transportation network, typically a mapped representation of the highway and public transit facilities of concern, along with a mapped representation of the population or employment activities that underlie travel demand. Defining a scenario can be considered the task of preparing the “inputs” to the travel forecasting exercise.
Each scenario is then subjected to a set of mathematical models that compare “demand” (as defined by land use and economic activity) with “supply” (as defined by the configuration of transportation facilities and services). Travel models keep track of thousands of people traveling at any given moment, as well as the purpose behind their trip, the personal attributes that guide their travel choices and the transportation conditions they encounter along the way. This is accomplished by structuring the models in a sequence that responds to changes in land use patterns, economic conditions, transportation operations and physical infrastructure.
Travel forecasts, themselves, are the “outputs” of these models. They include measures of the overall volume of travel, the modes and facilities used, and the time, distance and out-of-pocket costs of doing so.
The above introduction describes the principal approach to travel forecasting as it has developed in the United States since the 1950s; and has since evolved in a variety of directions and degrees of complexity. If you are new to travel forecasting methods, we recommend that you follow-up by exploring the two primary approaches in practice today by public planning agencies in the US.
Beyond these, this Travel Forecasting Resource wiki includes discussions of many different types of traffic forecasting methods at varying levels of complexity.