Emissions Modeling

Needs for Emissions Modeling

There are several needs for the modeling of transportation-related emissions in planning and policy analysis. In a broad sense, emissions modeling is conducted to develop emissions inventories. Historical and current emissions inventories are used by policy makers to assess emissions trends, understand the contribution of each emission source, develop strategies to reduce emissions, and track progress over time. Future emissions inventory forecasts are used to assist in the selection of appropriate emissions reduction strategies and to establish emissions reduction goals.

In the context of planning, emissions modeling is performed by transportation agencies to evaluate the potential impacts of transportation plans, programs, and projects on air quality in their areas. In the United States, this is required of agencies in nonattainment areas, which do not meet one or more of the National Ambient Air Quality Standards (NAAQS). The requirement, referred to as transportation conformity, aims to ensure that federal funding and approval are given to highway and transit projects that will not lead to new air quality violations, worsen existing violations, or delay the ability of an area to attain the standards.

Some transportation policies are partly driven by the desire to reduce emissions from on-road motor vehicles. For example, U.S. federal policies encourage construction of high occupancy vehicle lanes instead of general-purpose lanes in nonattainment areas. Many policies that are aimed at reducing traffic congestion also have a cobenefit of reducing traffic emissions. Examples include congestion pricing, car sharing, transit improvement, etc. Emissions modeling can be performed to analyze the emissions reduction benefits of these policies.

In addition, emissions modeling can play a role in the analysis of policies related to other issues as well as transportation. For example, emissions modeling is one of the key modeling steps required to determine the level of transportation-related air pollution near roadways. The modeling results can have implications for public health and land use policies. For example, sites with a high fraction of sensitive populations, such as schools and nursing homes, should be located far enough away from major highways.

Macroscopic Approach

Emissions from transportation sources can be modeled in many ways. For modeling at the macroscopic level, data input requirements are minimal and the modeling approach is not sophisticated. On the other hand, modeling results are sensitive to only a limited set of variables, which limits the range of policy questions that can be addressed. For example, at the national or state level a spreadsheet tool can be used to model carbon dioxide emissions based on fuel sale records. However, this method will not reveal, for instance, whether the reduction in carbon dioxide emissions is a result of reduction in vehicle miles traveled or improvement in vehicle fuel efficiency.

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