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

该项目将调查城市空间结构，包括人口密度，如何影响公共交通和私人车辆的相对碳效率。尽管公共交通经常被宣传为一种比驾车更可持续的乘客出行方式，但公共交通的碳效率在很大程度上取决于公共交通车辆载客量。在美国城市地区的许多地区，人口密度和公共交通乘客非常少，以至于公共交通每乘客英里排放的二氧化碳比汽车更多。该项目将提供新的信息和见解，以解决对提供城市交通和大都市形态有重大影响的一系列基本问题。该项目将为以下问题提供更好的答案：公共交通将以多大的密度拥有足够的交通乘客，从而比私家车对环境的危害更小？城市地区还有哪些其他空间特征和政策会影响交通的碳效率？紧凑型城市形态的积极影响是否在一系列人口密度中显著增加？该项目将根据不断变化的气候和其他环境条件，提出和估计碳效率密度阈值，从而加强对理想和可持续形式的人类住区的了解。该项目还将导致城市空间形态度量的发展，这些度量将有效地衡量城市区域水平的空间结构，并为综合土地利用和交通政策提供信息。项目成果将为通过促进更可持续的交通方式大幅减少运输部门温室气体排放所需的空间变化提供新的见解。将为今后广泛的基础和应用研究活动提供有形的项目产出，例如一套全面的空间指数和按交通方式估计的125个城市化地区的二氧化碳排放量。通过透视可持续城市形态的作用，该项目将提供重要的参考点，以帮助完善智能分区代码和以交通为导向的开发设计指南。城市地理学家和规划师为定义理想的人类住区规模或形式所做的努力由来已久。经济效率一直是最优城市规模和密度理论的主要动机，对环境可持续性的关注很少。在过境维持密度阈值的文献中，财务可行性也一直是一个普遍的标准。研究人员将在两个概念水平上探索城市化地区人口密度的碳效率阈值。他将确定使公共交通比平均载客车辆更具碳效率所需的最低人口密度，以及密度水平，当密度对公共交通的相对碳效率产生积极影响时，公共交通的相对碳效率将大幅增加。调查人员将使用2009年国家交通数据库和全国家庭旅行调查数据，估计美国125个最大的城市化地区按交通工具和私人车辆每乘客英里的碳排放量。他将为样本中的城市化地区开发人口密度和中心度指标，并将在测试是否存在碳效率密度阈值的同时探索密度和交通碳效率之间的非线性关系。

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.