The Clean Air Act requires that transportation investments be based on the most recent information that is available in order to protect public health over the long-term. As such, conformity determinations must be based upon the most recent planning assumptions in force at the time the conformity analysis begins (40 CFR 93.110(a)). The Environmental Protection Agency (EPA) and Department of Transportation (DOT) encourage state and local agencies to review and update their planning assumptions regularly. Areas are strongly encouraged to review and update planning assumptions at least every five years, especially population, employment, and vehicle registration assumptions. Areas with network-based travel models should review their assumptions and data used in model validation through the consultation process, and newer assumptions and data must be used whenever available. Conformity determinations that are based on assumptions that are older than five years should include written justification for not reviewing and updating assumptions at least every five years.

Title from title screen (viewed Feb. 12, 2009). Appendix B is on a World document accessible from the website. "December 2008." Includes bibliographical references. "EPA420-B-08-901."

This guidance document supersedes the January 18, 2001 guidance memorandum to address the July 1, 2004 transportation conformity final rule revisions on the use of latest planning assumptions in transportation conformity determinations.