Green Accessibility: Measuring the Environmental Costs of Space-Time Prisms in Sustainable Transportation Planning

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This project focuses on estimating the environmental cost of transportation-based accessibility. Accessibility is a central concept in transportation science, planning, and engineering. Accessibility refers to an individual's potential mobility and interactivity within a given geographic setting using transportation and communications technologies. Transportation scientists, planners, and engineers have a well-developed theory and methodology for measuring transportation-based accessibility: the space-time prism (STP), which is the envelope of an individual's possible travel paths between two geographic locations given limits on available time and travel speeds. The STP measures how well a transportation system provides mobility options that connect people to places they might want to go. Scholars have not considered the environmental costs of this potential mobility, however. Estimating the resource cost of actual mobility involves monitoring the resource consumption and/or emissions of a vehicle as it moves through geographic space. Calculating the environmental costs of a STP is difficult, however, because there are many potential paths, but only one will actually occur. This research project will extend the STP to include the environmental costs of accessibility. The investigators will improve theoretical understanding of the STP by developing methods for estimating the distribution of potential paths within the prism interior. They will develop practical measures of expected mobility costs for prisms in both continuous space and transportation networks, and they will evaluate the expected cost measures using empirical mobility data. They will develop geographic information system (GIS) software for applying prism benefit/cost measures in transportation planning and project evaluation, including a usability evaluation involving practicing transportation planners. Finally, they will create a framework for deploying STP cost/benefit measures in federally mandated transportation planning documents.This project will substantially advance scientific knowledge about human accessibility, a fundamental concept in transportation science, planning, and engineering as well as in other social and behavioral sciences. This interdisciplinary project will link fundamental concepts in the human sciences (accessibility) and physical science and engineering (resource consumption and waste emissions) with modern transportation planning practice, creating new transdisciplinary opportunities. Graduate and undergraduate students from geography, urban planning, and transportation engineering will be engaged in research that is relevant to basic scientific understanding as well as to a critical, real-world problem. The project will have potentially transformative effects on transportation as well as urban and sustainability science and planning. The extension of accessibility theory to environmental science will challenge the assumption in transportation science and planning that greater accessibility is a generally unmitigated benefit to individuals and society, thereby helping generate more nuanced and sophisticated approaches to the analysis, planning, and management of complex human-physical systems like transportation and cities.