The regional emissions analysis is the key analytic component of the transportation conformity process, and is conducted to demonstrate that transportation plans, Transportation Improvement Programs (TIPs), and projects are in conformity with the State Implementation Plan (SIP) for air quality. A critical factor that will influence what methods can be used for the regional emissions analysis is whether or not the region has a travel demand forecasting (TDF) model. The transportation conformity rule specifically requires that serious and above ozone and carbon monoxide nonattainment areas with urban population more than 200,000 use network-based travel models for the regional emissions analysis. For small urban and rural areas, and others not meeting these criteria, the conformity rule allows areas flexibility to conduct regional emissions analysis by either continuing the existing modeling practices of the MPO or by using "any appropriate methods" that account for vehicle miles traveled (VMT) growth and future transportation policies.
Many small urban and rural nonattainment and maintenance areas face challenges in conducting the regional emissions analysis. These areas often do not have TDF models to generate travel outputs required for use in emissions analysis. They also often have very limited data on VMT and speeds required for emissions analysis. As a result, existing small urban and rural areas have faced questions about what are appropriate methods for conducting a regional emissions analysis given limited data and tools.
The purpose of this document is to provide information on methodologies and adjustment techniques that have been used for regional emissions analysis in several small urban and isolated rural nonattainment and maintenance areas. The methodologies described in this document were identified through a research effort that involved a literature review and contacts with staff from over twenty State Departments of Transportation (DOTs) and metropolitan planning organizations (MPOs) that conduct conformity analysis. This document describes and assesses methodologies, and is intended to help small urban and rural areas gain a better understanding of some of the procedures that have been used for conducting regional emissions analysis.
This document is designed as a "menu" of methodologies and adjustment techniques that can be used in small urban and rural areas for regional emissions analysis. An important theme behind this document is that a variety of methods are available and appropriate for different circumstances. This document is not intended to direct a specific methodology to be used in a particular location. The methodology that is ultimately used should be determined through the interagency consultation process, and should reflect considerations appropriate for the nonattainment or maintenance area. The methodologies profiled in this document are not necessarily comprehensive; other methods may be feasible or appropriate.
The document is organized as follows:
The document is divided into three main sections, which relate to specific inputs required for conducting the regional emissions analysis:
A typical user of the document should review methods within each of these three sections since each addresses a necessary component of the emissions analysis process. Moreover, how VMT is treated will have impacts on how speeds must be treated.
Section 5 of the report contains a summary, and Section 6 provides resources for additional information on regional emissions analysis and the conformity process.
Although not required to use TDF models, many small urban areas do have TDF models, and use them to conduct conformity analysis. A number of States have also developed statewide TDF models (e.g., Maine, Michigan, Oregon), which are used to estimate VMT for higher-order roadway classifications. The methodologies that can be used in these areas to forecast VMT and speeds will differ considerably from those in areas without models. As such VMT and speed methodologies are addressed separately for areas with and without TDF models. In cases where a TDF model is available, techniques are often used to adjust the outputs of the models to reflect local conditions and to ensure that the results can be used appropriately for emissions analysis. In addition, methods are sometimes needed to address "donut" areas or specific functional roadway classifications that are not addressed by the TDF model.
The typical user of this document can skip to the appropriate subsections of the document depending on whether or not a TDF model is available.
Each methodology is presented in a standard format for easy reference. For each methodology, the document briefly describes the method, discusses where the method is most applicable, and provides information on advantages and limitations of the approach, based on ICF Consulting's assessment. It also identifies a sample location where the methodology has been applied.
Each method is assessed on a qualitative scale (from low to high) across four criteria, in order to help the user determine the applicability of each:
Policy Sensitivity - How sensitive are the results of the methodology to changes in highway investments, transit investments, or other policies? Methodologies that take into account the effect of transportation decisions will score "high" (e.g., a VMT methodology that predicts lower VMT based on increased transit investments and associated transit service improvements would exhibit high policy sensitivity). In contrast, methods that predict the same results regardless of relevant policy changes will score "low" (e.g., speed tables that predict the same speed for traffic regardless of a large investment in a major corridor signal coordination project would be exhibit low policy sensitivity).
It is important to point out that simply because a methodology scores "low" on technical robustness or policy sensitivity does not mean that the methodology is inferior or should be avoided. In some cases, a relatively simple methodology may be the most appropriate, and it may not be worth the additional effort or cost to use a more complex methodology if the results are not expected to be substantially different. The specific circumstances in the region will determine which approaches are most appropriate.
 In California, emissions factors are developed through the EMFAC model rather than through MOBILE. Although the MOBILE model is referred to through this report, many of the same methodologies can also be applied in developing inputs to EMFAC.
 A donut area is a geographic area that falls within the boundary of a nonattainment or maintenance area that contains a metropolitan area, but falls outside of the metropolitan planning area boundary. Emissions in donut areas in most cases must be included in the metropolitan area regional emissions analysis for the Plan and TIP.
 It should be noted that EPA's MOBILE6 model is currently required for use in conformity analysis for all states outside of California. Several of the methodologies identified through this research were applied using previous versions of EPA's MOBILE model, EPA's PART5 model to estimate particulate matter, or the EMFAC model in California. These methodology descriptions, in some cases, have been adapted slightly to reflect procedures that could be applied using MOBILE6.