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Transportation Conformity Domestic Scan Report: Use of Latest Planning Assumptions and Transition to MOBILE6

Chapter 1 - Introduction

Purpose of the Scan Project

The Federal Highway Administration (FHWA) transportation conformity domestic scan was initiated to identify good practices among a select number of nonattainment and maintenance areas for meeting the transportation conformity requirements of the Clean Air Act. The scan focused on the use of latest planning assumptions in the conformity process and also highlighted areas' efforts to address data issues associated with the transition to the use of EPA's most recent motor vehicle emissions model, MOBILE6. The domestic scan was modeled after FHWA's long-standing International Technical Scanning Program; an effort to share information between the United States and other countries on issues relating to transportation policy, planning, design, construction, maintenance and technology. In addition, the transportation conformity scan project was developed in response to State and regional agency requests to identify and share information on how areas are meeting various transportation conformity requirements.


A key requirement of transportation conformity is the use of latest planning information and models in the development of travel and emissions estimates. Specifically, the Clean Air Act section 176(c) states that conformity determinations must be "based on the most recent estimates of emissions, and such estimates shall be determined from the most recent population, employment, travel and congestion estimates." After nearly ten years of experience with the transportation conformity process, a great deal has been learned about how to make the process more efficient and many good practices have been developed that can be shared with transportation conformity stakeholders.

Many of the good practices described in this report, however, are not specifically required by the Clean Air Act or transportation conformity rule. The scan team decided that the inclusion of these practices were useful and provide tools for meeting, and in some cases advancing the state of the practice beyond the minimum requirements for using latest planning assumptions in conformity. Under the current conformity rule, nonattainment and maintenance areas must use the most recent planning assumptions that are available at the time a conformity determination is made. The joint DOT/EPA January 18, 2001, guidance1 clarifies the Clean Air Act and conformity rule requirement (40 CFR 93.110) for using the latest planning information in conformity determinations. The January 2001 guidance encourages areas to review and update assumptions, especially population, employment and vehicle registration assumptions, every 5 years. The guidance also emphasizes the importance of the interagency consultation process and its role in determining the most recent available information for use in conformity.

The conformity rule also requires that conformity determinations must be based on emissions estimated by the latest emissions model (40 CFR 93.111). On January 29, 2002, EPA officially released the latest emissions model, MOBILE62. MOBILE6 is the required model to be used for mobile source emissions estimates for SIP and conformity analysis in all States except California3. EPA provided a 2-year grace period for areas to phase in this model for both SIP and conformity purposes. This requires that all conformity analyses started subsequent to January, 29, 2004 use MOBILE6. Many areas are in the process of transitioning their regional emission analysis to MOBILE6. The scan team decided to also include a review of how the scan sites are transitioning to MOBILE6 with a specific interest in what and how local data is being collected, analyzed, and prepared for the MOBILE6. The decision of documenting the experience of transitioning into MOBILE6 in the six sites is two-fold. First, MOBILE6 has the capability to incorporate much more detailed data than earlier models, and a few of the scan areas we visited have expanded their data collection efforts to take advantage of the added capabilities of the model. Second, some of these areas are in the process of testing the sensitivity of whether the national defaults or local data should be used in the model4. The scan team believes this experience may be helpful to others that are contemplating the same process.

Selection of Scan Sites

Six areas were selected for the transportation conformity domestic scan. Table 1.1 below shows the sites visited and dates.

Table 1.1 Conformity Scan Sites
Conformity Scan Sites Date Visited
Harrisburg, PA August 15, 2002
Raleigh, NC Sept.10, 2002
Dallas-Ft. Worth, TX Nov. 4, 2002
Atlanta, GA Nov. 5, 2002
Tampa, FL April 29, 2003
Nashville, TN May 1, 2003

These six sites were selected to represent large, medium and small MPOs but facing similar air quality issues, and are from predominantly the South and Southeast regions. With the pending designations of new nonattainment areas for the 8-hour ozone (O3) and fine particulate (PM2.5) air quality standards, these States are logical areas in which to focus, as a number of new nonattainment areas are anticipated in these states.

Some conformity requirements, such as travel demand modeling requirements, differ based on nonattainment classifications and population of areas. Therefore it was also decided to select sites that represent different ranges of nonattainment classifications.

Selection of Scan Team

A group of conformity practitioners with diverse experience in the conformity process were invited to become members of the scan team. Together they represent the Federal government, and State and local transportation and air quality agencies, in both policy development and technical analysis. The ability of the scan team to share personal experience in working with conformity in their respective areas was considered an asset and was clearly appreciated in those sites visited. The geographic diversity of the areas represented by the scan team is also important because transportation conformity and air quality concerns result from different issues in different areas of the country. Table 1.2 lists the members of the Conformity Scan Team. Appendix A includes brief biographies of team members.

Table 1.2 Conformity Scan Team
Regina Aris Baltimore Metropolitan Council, Baltimore, MD
Charles Baber* Baltimore Metropolitan Council, Baltimore, MD
Kip Billings Wasatch Front Regional Council, Salt Lake City, UT
Eddie Dancausse FHWA-NC Division, Raleigh, NC
Gary Dolce EPA-Office of Transportation and Air Quality, Ann Arbor, MI
Cecilia Ho FHWA-Headquarters, Washington, D.C.
Gary Jensen FHWA-Headquarters, Washington, D.C.
Susan Lee FHWA-Headquarters, Washington, D.C.
Cynthia Marvin California Air Resources Board, Sacramento, CA
Sarah Siwek Sarah J. Siwek & Associates, Inc., Los Angeles, CA
Lynn Soporowski Kentucky Transportation Cabinet, Frankfort, KY
Angela Spickard EPA-Office of Transportation and Air Quality, Ann Arbor, MI
Spencer Stevens FHWA-PA Division, Harrisburg, PA
Tianjia Tang FHWA - Resource Center, Atlanta, GA

Overview of Scan Format

Prior to each visit, background information was sent to scan team members to help facilitate understanding of the transportation conformity and air quality issues in the area to be visited and the context for air quality issues in relationship to transportation planning. In addition, a list of topics for discussion (See Appendix B) was sent to each of the participants to be involved in the discussions at each site. This helped focus the conversations on the two specific topics of interest.

The scan team spent a day and a half at each site. On the first half-day of each visit, the scan team met with FHWA Division Office staff and the MPO Director. The second half-day of each visit included a "round table" discussion with the scan team and representatives involved in the respective area's interagency consultation process, including the MPO, state DOT, State and local air agencies, and FHWA/FTA and EPA Field Offices. See Appendix C for a listing of the participants at each site. Upon completion of each scan visit, a summary of each visit was developed, reviewed by the scan team, and posted on the FHWA Transportation Conformity Community of Practice website. ( This final report on the domestic scan provides a synthesis of findings from all six scan site visits.

1 January 18, 2001, memorandum entitled, "Use of Latest Planning Assumptions in Conformity Determinations" issued by FHWA, FTA and EPA.

2 See EPA's MOBILE6 website to download a copy of the "Technical Guidance on the Use of MOBILE6 for Emission Inventory Preparation" which was issued in January 2002. The MOBILE6 website is located at: The current version is MOBILE6.2.

3 The current EPA approved emissions model for California is called EMFAC2002.

4 MOBILE6 national defaults are commonly used in emissions modeling when no better state or local data exists (e.g., MOBILE6 fleet defaults may be better than older local fleet data), However, EPA encourages areas to use local data when available.

Updated: 10/20/2015
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