Relationships Between Urban Population Densities and the Carbon Efficiency of Public Transit and Private Vehicles

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

The project will investigate how urban spatial structure, including population density, affects the relative carbon-efficiencies of public transit and private vehicles. Although public transit is often promoted as a more sustainable passenger travel mode than driving, transit's carbon efficiency largely depends on how many passengers transit vehicles carry. In many parts of urban areas in the U.S., population densities and transit ridership are so low that transit emits more carbon dioxide per passenger mile than automobiles. This project will provide new information and insights to address a range of fundamental questions that have significant implications regarding the provision of urban transportation and metropolitan form. Among the questions for which the project will provide better answers are the following: At what density will mass transit have enough transit riders to be less environmentally hazardous than private cars? What other spatial characteristics and policies in urban areas influence the carbon-efficiency of transit? Is there a range of population densities in which positive impacts of compact urban form dramatically increase? This project will enhance knowledge about desirable and sustainable form of human settlements by proposing and estimating carbon-efficient density thresholds in light of changing climatic and other environmental conditions. The project also will result in the development of urban spatial form metrics that will effectively measure urban area level spatial structure and inform integrated land use and transportation policy. Project findings will yield new insights into the spatial changes that are needed to significantly reduce transportation sector greenhouse-gas emissions by promoting more sustainable transportation modes. Tangible project outputs, such as a comprehensive set of spatial indices and estimated carbon dioxide emissions by transportation mode for the 125 urbanized areas will be available for a broad range of future basic and applied research activities. By putting the role of sustainable urban form into perspective, this project will provide important reference points to assist in the refinement of smart zoning codes and design guidelines for transit-oriented developments. Efforts by urban geographers and planners to define an ideal size or form of human settlements have a long history. Economic efficiency has been dominant motivation for optimal city size and density theories, with little attention paid to environmental sustainability. Financial feasibility has also been a single prevailing criterion in the literature of transit sustaining density thresholds. The investigator will explore carbon-efficient thresholds of urbanized area population density at two conceptual levels. He will determine minimum population densities that are required to make public transit more carbon-efficient than average occupancy vehicles as well as density levels at which positive density impacts on relative carbon efficiency of public transit dramatically increase. The investigator will estimate carbon emissions per passenger mile by transit modes and private vehicles in the 125 largest urbanized areas of the U.S. using 2009 National Transit Database and National Household Travel Survey data. He will develop population density and centrality metrics for the urbanized areas in the sample, and he will explore the non-linear relationship between density and transit carbon-efficiency while testing for the presence of carbon-efficient density thresholds.