(a) A State or eligible Tribe (a federally recognized tribal government determined to be eligible to submit a TIP under 40 CFR 49.6) may submit to the Environmental Protection Agency (EPA) a revision to its applicable implementation plan which contains criteria and procedures for assessing the conformity of Federal actions to the applicable implementation plan, consistent with this section and 40 CFR part 93, subpart B.

(b) Until EPA approves the conformity implementation plan revision permitted by this section, Federal agencies shall use the provisions of 40 CFR part 93, subpart B in addition to any existing applicable State or tribal requirements, to demonstrate conformity with the applicable SIP or TIP as required by section 176(c) of the CAA (42 U.S.C. 7506).

(c) Following EPA approval of the State or tribal conformity provisions (or a portion thereof) in a revision to the applicable SIP or TIP, conformity determinations shall be governed by the approved (or approved portion of) State or tribal criteria and procedures. The Federal conformity regulations contained in 40 CFR part 93, subpart B would apply only for the portion, if any, of the part 93 requirements not contained in the State or Tribe conformity provisions approved by EPA.

(d) The State or tribal conformity implementation plan criteria and procedures cannot be any less stringent than the requirements in 40 CFR part 93, subpart B.

(e) A State's or Tribe's conformity provisions may contain criteria and procedures more stringent than the requirements described in this subpart and part 93, subpart B, only if the State's or Tribe's conformity provisions apply equally to non-Federal as well as Federal entities.

(f) In its SIP or TIP, the State or Tribe may identify a list of Federal actions or type of emissions that it presumes will conform. The State or Tribe may place whatever limitations on that list that it deems necessary. The State or Tribe must demonstrate that the action will not interfere with timely attainment or maintenance of the standard, meeting the reasonable further progress milestones or other requirements of the Clean Air Act. Federal agencies can rely on the list to determine that their emissions conform with the applicable SIP or TIP.

(g) Any previously applicable SIP or TIP requirements relating to conformity remain enforceable until EPA approves the revision to the SIP or TIP to specifically remove them.