Travel Demand Models

Land-use models are frequently integrated with transportation models because of the strong interdependencies between these phenomena. From early settlements along navigable water bodies to modern developments adjacent to freeway interchanges, land-use patterns have historically been linked to the existing modes and routes of transportation. The relationship between land use and transportation, and the need to plan for them in an integrated fashion, has been widely recognized in the research community as well as by the US Federal Highway Administration. For example, the Intermodal Surface Transportation Efficiency Act of 1991 and the Transportation Equity Act for the Twenty First Century of 1997 require state or regional transportation agencies to consider the effect of transportation infrastructure development on land-use patterns and consistency with land-use plans in order to receive certain types of federal transportation funds. Travel-demand modeling is frequently used to address this need by metropolitan planning organizations, the regional entities that administer much of the federal highway funding.

Travel-demand models are frequently employed to evaluate transportation infrastructure performance, investment alternatives, and air quality impacts under future land-use and transportation scenarios. The area of interest is represented as a discrete number of zones, often called traffic analysis zones, and the model aims to quantify travel activity between zones. Within these zones are features of attraction (e.g., schools, stores) that generate incoming and outgoing travel. Estimating travel demand typically follows a four-step process which includes trip generation, trip distribution, modal split, and traffic assignment. The trip generation step quantifies the number of incoming and outgoing trips from each analysis zone based on land-use patterns, and classifies these trips according to their purpose (e.g., home to work, home to shopping). Trip distribution assigns the incoming and outgoing travel from the trip generation step to specific zones. The modal split step estimates the number of trips by mode of transport based on the socioeconomic characteristics of the trip maker. Finally, the traffic assignment identifies the route for each trip.

Models of this type are generally referred to as Lowry-type models, after Ira Lowry's land-use and transportation model developed for the Pittsburgh, PA, region in the 1960s. The model is constrained by exogenous employment and population control totals. The regional distribution of land uses and the transportation infrastructure that links them determines the travel cost between locations. The intent of the model is to quantify travel demand for existing and proposed transportation infrastructure based on current land-use patterns. Travel-demand models as decision making tools can be used to ensure the provision of adequate transportation infrastructure and to estimate air quality indicators. When coupled with a land-use model, however, the ability to simulate dynamic interactions between transportation infrastructure, travel demand, and human decision making becomes a powerful tool for better understanding emerging landscape patterns and their resulting ecological impacts.

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