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travel times in the network, which will in turn become inputs for the gravity model. For the construction phase, changes in accessibility can include delays that can be estimated by the number of lanes lost. The construction delay can also be estimated through expert judgment based on past experience with similar projects. For the postconstruction period, changes in accessibility will likely be available from other elements of the project analysis. The information on changes in accessibility can then be used to forecast the change in link travel times in the overall network. Forecasted link travel time changes, for both the construction and post- construction time periods, can be used to obtain a gravity model assessment of the spatial distribution of the economic impacts of a highway project. Results and their presentation. The primary results obtained from applying gravity models is an assessment of how businesses in a particular area of the community would fare if a major transportation project were undertaken. The analysis can be carried out to estimate competitive effects during the construction phase, as well as when the project is completed. As indicated in Figure 9-3, it is possible to superimpose a spatial depiction of changes in competitiveness on socioeconomic data. This facilitates an assessment of the economic development effects on low- income or minority activity spaces of the community. Assessment. In agencies with readily available transportation planning models, the method for assessing environmental justice impacts using the gravity model is quite simple and straightforward to implement. Agencies with limited resources, however, may not be able to utilize this method. The method works well with large transportation projects to identify which areas of a community would experience gains or losses but is sometimes not sensitive enough for smaller ones. This is due to the aggregate nature of the analysis, with data typically aggregated to TAZs or similarly sized geographic observations. Of course, if small-scale geographic impacts are important, one could collect data for smaller areas. In assessing distributive impacts, the unit of analysis should be internally homogenous in racial, ethnic, or income distribution. If this is not the case, variations within a geographic area will possibly have environmental justice implications that would not be illuminated by the analysis. If the geographic observations are not internally homogenous, additional studies on microlevel distributive effects within areas of special concern can be used as a supplement to further clarify environmental justice issues. Another important problem that cannot be addressed explicitly by the gravity model is the impact of new development. New highways can attract new development, which could in turn affect the distribution of sales. The gravity model, however, treats retail businesses as exogenous and therefore cannot capture this aspect of the distributive impacts of transportation projects. The problem can be alleviated if new development can be forecast. The forecast future retail floor space can then be input into a gravity model. This will yield results whose reliability will depend on the quality of the forecast of changes in the location of business activity. RESOURCES 1) Burkhardt, Jon E., James L. Hedrick, and Adam T. McGavock (Ecosometrics, Inc.). 1998. TCRP Report 34: Assessment of the Economic Impacts of Rural Public Transportation. Washington, DC: Transportation Research Board, National Research Council. 228
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This report examines the economic effects of selected rural public transportation services at the local level through case studies. It provides practical examples of how to assess effects associated with the introduction or expansion of public transportation services in rural areas. 2) Cambridge Systematics, Inc., Robert Cervero, and David Aschauer. 1998. TCRP Report 35: Economic Impact Analysis of Transit Investments: Guidebook for Practitioners. Washington, DC: Transportation Research Board, National Research Council. This report provides guidance on selecting methods to conduct analysis of the economic and land development effects of transit investments. It reviews the major methods and shows their application through case studies. 3) Forkenbrock, David J., Thomas F. Pogue, Norman S. J. Foster, and David J. Finnegan. 1990. Road Investment to Foster Local Economic Development. Iowa City: University of Iowa, Public Policy Center. A detailed presentation of the conceptual relationship between transportation investment and economic development is contained in this monograph. The relationship is explored in an analysis of postinvestment effects of businesses that benefited by specific road projects. 4) Hagler Bailly Services, Inc. 2000. "Guidance on Using Economic Analysis Tools for Evaluating Transport Investments." National Cooperative Highway Research Program, Project 2-19(2) Contractor Final Report. Washington, DC: Transportation Research Board, National Research Council. This NCHRP report discusses research and application of existing techniques for measuring economic development and productivity effects of transportation projects. It discusses the appropriate use of existing tools, including their usefulness, reliability, and data requirements. It is designed to help analysts select appropriate techniques given their unique needs, data constraints, and staffing expertise. Case study examples are provided. 4) Weisbrod, Glen, and Burton Weisbrod. 1997. "Assessing the Economic Impacts of Transportation Projects: How To Choose the Appropriate Technique for Your Project." Transportation Research Circular 477. Washington, DC: Transportation Research Board, National Research Council. This circular, sponsored by the Transportation Research Board Committee on Transportation Economics, is a concise primer on how to best assess economic effects of transportation projects. It is designed to provide the reader with guidelines for (1) identifying the types of economic effects most relevant for decision-making, (2) defining the appropriate evaluation perspective, and (3) selecting techniques to be used for analysis and presentation of findings. 5) Wilbur Smith Associates, Benjamin J. Allen, C. Phillip Baumel, David J. Forkenbrock, and Daniel Otto. 1993. Guide to the Economic Evaluation of Highway Projects. Ames, IA: Iowa Department of Transportation. This guidebook identifies methods by which economic analysis can be used to help decision- makers select highway projects and project types that would produce net economic benefits. It explains how the included methodologies work and discusses how to ensure that they are applied so as to produce results that are consistent and fair. 229