This chapter implements RCW 70.94.037 of the Washington Clean Air Act (chapter 70.94 RCW). The law requires the departments of ecology and transportation to develop criteria and guidance for demonstrating and assuring conformity of transportation plans, programs, and projects to the purpose of the state implementation plan for attaining and maintaining the national ambient air quality standards and meeting the requirements of the federal Clean Air Act (42 U.S.C. 7401) as amended. This chapter is jointly adopted by the departments of ecology and transportation and can be amended only by agreement between the departments. This chapter sets forth minimum requirements for evaluating transportation plans, programs, and projects for conformity with the purpose and intent of state implementation plans for air quality. This chapter clarifies state policy and procedures to achieve national ambient air quality standards, foster long-range planning for attainment and maintenance of those standards, provide at least as stringent requirements as the federal conformity regulation (40 C.F.R. Part 51 Subpart T), provide a basis for evaluating conformity determinations, and guide state, regional, and local agencies in making conformity determinations.