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.

The objective of this research was 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.  The final report was published as NCHRP Report 535.