Transportation Conformity is required under Section 176(c) of the Act to ensure that Federally supported highway, transit projects, and other activities are consistent with (conform to) the purpose of the approved State Implementation Plan (SIP). Transportation Conformity applies to areas that are designated nonattainment and those areas redesignated to attainment after 1990 (maintenance areas), with maintenance plans developed under section 175A of the Act for the following transportation-related criteria pollutants: Ozone, particulate matter (PM 2.5 and PM 10), carbon monoxide (CO), and nitrogen dioxide (NO 2). Conformity to the purpose of the SIP means that transportation activities will not cause new air quality violations, worsen existing violations or delay timely attainment of the relevant National Ambient Air Quality Standards (NAAQS). The Federal Transportation Conformity regulations (Federal Rule) are found in 40 CFR part 93 subpart A, and provisions related to conformity SIPs are found in 40 CFR 51.390.

Based on air quality monitoring data gathered between 2006 and 2009, Southwest Indiana (Evansville area) was re-designated to a maintenance area for the annual PM2.5 1 National Ambient Air Quality (NAAQS) by the USEPA on September 27, 2011 (FR Vol. 76, No. 187). The southwest Indiana PM2.5 maintenance area includes the counties of Vanderburgh and Warrick in the MPO boundary area. The attainment maintenance area also includes a donut area adjacent to the MPO boundary comprised of Dubois County, the township of Montgomery in Gibson County, Washington Township in Pike County and Ohio Township in Spencer County. Figure 1 shows the maintenance area designated by USEPA for PM2.5