During the last three decades, a renewed interest has been growing in the effects that urban form has on the travel behavior of residents and workers. Agencies typically responsible for effecting research into or applications of this subject include Metropolitan Planning Organizations (MPOs); state departments of transportation (DOTs); state and federal agencies responsible for setting or enforcing air quality standards; transit operators and researchers; environmental advocacy organizations; and a cross-section of citizens concerned about traffic congestion, sustainable living arrangements, human health, and various aspects of the environment. A considerable amount of research has been directed towards this topic from all of these disciplines, with each research item imparting its own "spin" on technical methods, hypotheses, input information, and conclusions.
The focus of this synthesis work is on the specific, quantifiable relationship between urban form and pollutant emissions from mobile sources. While research has been conducted prior to 1990, that year marks a major departure point for research into this topic since it was also the year that major amendments were made to the Clean Air Act. These amendments closely tied the results of emissions forecasts to both projects and programs (transportation plans, programming documents, and transportation projects) and served to catalyze additional research into air emissions and transportation conformity. Partially as a result, the pace and quality of research into this subarea of the urban form/emissions topic increased substantially during the last 10 years. During nearly the same time period, research has been conducted on qualifying and quantifying the relationship between urban form and on-road emissions of so-called greenhouse gasses (mainly carbon dioxide and methane). It is important to recognize the distinction between these two major fields of domestic study: greenhouse gases are not regulated in the United States, but the criteria pollutants described in the Clean Air Act and its 1990 amendment (CAA and CAAA90, respectively) are regulated through the transportation conformity process and other mechanisms. For the purposes of this project, the typically area-based urban form-greenhouse gas relationship was secondary to the quantification of emissions of regulated, mobile source criteria pollutants, and therefore is not generally discussed in this study. The criteria pollutants created in significant quantities by mobile sources - and therefore most relevant to this project - are particulate matter, nitrogen dioxide, ozone, and carbon monoxide.
The principle purpose of this report is to provide information on past and current attempts at quantifying emissions benefits from changes in land use strategies to identify state-of-the-practice methods that can be applied by end-user communities and refined through applied and theoretical research. Several products were produced that are expected to be of use to both application-oriented staff (e.g., MPOs) and researchers:
A basic, two-stage framework for articulating how land use and development patterns can affect mobile source emissions;
A comprehensive literature review of a variety of research papers on the topics of urban form, travel behavior, and mobile source emissions;
A critical review of research and applied methods from across the country, including public agencies, private entities, and research institutions; and
A number of tools that end-user communities and researchers can use to conduct and apply investigations into land use and emissions benefits, notably sections on recommended methods, elasticities/threshold values, and suggestions for additional research.
This project consisted of four efforts, each of which is briefly summarized below. FHWA staff reviewed interim deliverables during the project.
Task 1: Conduct Literature Search. The Research Team reviewed 46 research papers and reports that examined the relationships between land use, travel behavior, and emissions. These were summarized according to a consistent template and evaluation criteria applied by the Research Team. The resulting detailed bibliography was subsequently integrated into a vertically-searchable database and word processor formats. An initial evaluation of these methods was also provided in Technical Report #1.
Task 2: Analysis of State-of-the-Practice Methods. The Research Team conducted interviews of several agencies, and developed 11 detailed case studies of situations where an agency had quantified the emissions changes from potential land use alterations. Each case study is comprised of the context of the analysis, a brief summary, and a detailed description of the method(s) used by the agency conducting the analysis.
Task 3: Directions for Further Research. Based upon evidence from case studies, interviews and professional experiences, the Research Team identified a number of areas that could be further developed from a theoretical, educational, or application standpoint. These future improvements are viewed from the vantage of the practical needs of the end-user communities.
Task 4: Reporting. The Research Team developed a draft report for FHWA. Upon review of this draft, comments were received from both FHWA and EPA. These comments were incorporated into a final project report.
