The issue of greenhouse gas (GHG) emissions from transportation sources is likely to be addressed in key national legislation in coming years, for example through passage of a national climate and energy bill as well as potential provisions in the next surface transportation authorization bill. Such new legislation, like the existing Clean Air Act (as amended) could establish targets for emissions reduction, procedures for establishing such targets, or methods for judging whether strategies for reducing emissions are meeting targets. Current air quality regulations applicable to transportation rely substantially on ensuring that estimated total pollution emissions in particular regions are in conformity with budgets established in state implementation plans. Other proposed approaches include uses of incentives, cap-and-trade strategies, and establishment of carbon budgets.

Managing GHG emissions from transportation sources will entail analysis and decision making at several levels of planning and possibly changes in the underlying process currently used in transportation system planning. State departments of transportation (DOTs) currently hold primary responsibility for developing and managing the highway system, but metropolitan planning organizations (MPOs) play a major role in local transportation planning. Other agencies at local, state, and national levels are also involved in the planning process. These various agencies could be significantly affected by new legislative and regulatory initiatives intended to address GHG.

Some DOTs and MPOs have begun to develop their own analysis techniques for dealing with GHG, climate change, and other emerging air-quality issues.  These techniques and public expectations about the types of analysis and regulation that are appropriate for the transportation sector are likely to vary somewhat from place to place, as many factors important to management strategy are beyond the control of DOTs and MPOs, including geographical settings, institutional structures for implementation, and available mechanisms for enforcement of vehicle fuel efficiency and pollution emissions standards. Many observers suggest nevertheless that a national-level policy for achieving mobile-source GHG-reductions could provide a basis for ensuring consistency of objectives, range of strategies to be considered, scale of analyses, and computational models to be used in developing plans for managing GHG emissions from transportation sources.

Developing such a policy presents major challenges. For example, many DOTs and MPOs have found that the conformity-based procedures developed to enforce provisions of the Clean Air Act are costly, time-consuming, and at best produce only marginal improvements in projected air quality. The computational models developed to support conformity analyses have proven to be difficult to adjust to represent the impact of vehicle speeds, cleaner engines, cleaner fuels, and other changes in technology that are likely to produce the greatest reductions in air pollution emissions. Other approaches to transportation system planning and management to achieve air-quality goals may present comparable challenges.

To assist agencies in meeting these challenges, the National Cooperative Highway Research Program (NCHRP) undertook Project 20-24(64) to assess within a common framework the various mechanisms for that may be proposed for evaluating GHG-reduction strategies and integrating GHG objectives into transportation decision making.  The NCHRP 20-24 project series was initiated in 1988 to address problems experienced by top management of state departments of transportation (DOTs).  These research studies are requested by the American Association of State Highway and Transportation Officials (AASHTO); as of January 2010, more than 75 specific projects have been undertaken in finance, human resources, organization structure, strategic planning, leadership, business practices, and other areas of direct concern to high-level DOT executives.  This report is the final product of NCHRP Project 20-24(64).

The project’s objective was to provide a factual basis for judging the merits of alternative methods that DOTs and MPOs can use for managing GHG emissions from transportation. The project was intended to help policy makers to understand (a) how these alternative approaches to GHG emissions would affect states and metropolitan areas, (b) what approaches may be most effective for evaluating mobile-source GHG emission-management strategies, and (c) what particular tools are available to support implementation of these alternative approaches.

The research team worked under the guidance of the NCHRP project panel to describe and then assess a range of alternatives for addressing mobile-source emissions reductions such as application of regional emission reduction targets, incentives or penalties tied to such performance standards, and cap-and-trade mechanisms.  Workshops with officials from DOTs, MPOs, and other agencies helped the team to ensure that the alternatives and their assessment would be presented in terms likely to be useful to agency officials faced with developing and responding to GHG-reduction strategies in their regions, considering potential costs, uncertainties, and effectiveness of the alternatives.  The research considered variations among regions and availability of analysis tools for estimating the impact of transportation system management options on GHG.