The National Academies

NCHRP 25-21 [Completed]

Predicting Short-Term and Long-Term Air Quality Effects of Traffic-Flow Improvement Projects

  Project Data
Funds: $375,000
Research Agency: Dowling Associates
Principal Investigator: Richard Dowling
Effective Date: 10/19/1999
Completion Date: 4/30/2003

Background: With the passage of the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA) and its reauthorization, the Transportation Equity Act for the 21st Century (TEA-21), increased emphasis has been placed on informed decision making regarding the full range of environmental, system performance, financial, and other implications of statewide and metropolitan transportation plans and programs. A major component of providing accurate impact assessments centers around effective data collection and analytic methods to support decisionmakers.

The total air quality effects of transportation projects, especially those designed to improve traffic flow, are not fully understood. Projects may result in beneficial or detrimental impacts over the short or long term. For example, traffic-flow improvement projects may have a short-term air quality benefit by reducing congestion and increasing speed yet have a negative effect by facilitating additional travel. Also, transportation actions such as high-occupancy vehicle (HOV) projects, tolling strategies, and reduction in parking availability may have long-term air quality benefits by reducing trips and vehicle miles of travel (VMT), yet might make air quality worse in the short term by increasing congestion and queuing. Research is needed to improve the information available to support decision making in project evaluation, selection, and priority programming. Further, more accurate and objective information is needed by transportation decisionmakers regarding the full range of effects and impacts associated with traffic-flow improvement projects over the life of those projects.

Objective: The objective of this research is to develop and demonstrate, in case study applications, a methodology to predict the short-term and long-term effects of corridor-level, traffic-flow improvement projects on carbon monoxide (CO), volatile organic compounds (VOCs), oxides of nitrogen (NOx), and particulate emissions (PM). The methodology should evaluate the magnitude, scale (such as region-wide, corridor, or local), and duration of the effects for a variety of representative urbanized areas.

Tasks: This research will be accomplished through the following tasks: (1) Conduct a review of transportation and air quality literature to determine previous and current research studies that will support the objectives of and provide tools for the research. Prepare an annotated bibliography of the relevant literature. (2) Devise a methodology to predict short-term (less than 5 years) and long-term (more than 10 years) air quality effects of completed traffic-flow improvement projects. The methodology should evaluate those effects at local, corridor, and regional scales. Projects may include, for example, added freeway lanes, arterial widenings, intersection channelization, access management, HOV lanes, signal coordination, transit improvements, ramp metering, and park and ride lots. The methodology should include consideration of secondary effects of traffic-flow improvements, including possible changes in emissions resulting from project impacts on (1) safety and accessibility for pedestrians, bicyclists, and transit users; and (2) land use. To the extent possible, the methodology should be designed to use data sources commonly available in the transportation planning process. (3) Develop, for panel review and comment, criteria to select case studies and identify a variety of project types and urbanized areas (e.g., high- and low-growth areas and areas with heavy and light congestion) for which there are available appropriate data. Present, for inclusion in the interim report prepared in Task 4, potential case study project types and urbanized areas that meet the criteria developed in this Task. (4) Prepare an interim report covering the work performed and the findings from Tasks 1 through 3. Based on those findings, recommend any necessary work plan modifications for panel review and approval. Obtain panel approval for project continuation. (5) Based on guidance obtained from the panel, select a variety of traffic-flow improvement projects and test and validate the methodology. Identify any deficiencies in the analytical approach. Summarize findings and present recommendations for methodology improvement in a brief technical memorandum for review by the panel. (6) Based on the results of Task 5 and comments from the panel, refine the methodology as needed. (7) Document the results of the research, including both the methodology and the results of the case studies in the final report. Include a user guide for the analytic approach as an appendix to the final report.

Status: The project is completed.

Product Availability: The final report has been published as NCHRP Report 535.

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