Section 93.159 - Procedures for conformity determinations of general Federal actions(a) The analyses required under this subpart must be based on the latest planning assumptions. (1) All planning assumptions must be derived from the estimates of population, employment, travel, and congestion most recently approved by the MPO, or other agency authorized to make such estimates, where available.(2) Any revisions to these estimates used as part of the conformity determination, including projected shifts in geographic location or level of population, employment, travel, and congestion, must be approved by the MPO or other agency authorized to make such estimates for the urban area.(b) The analyses required under this subpart must be based on the latest and most accurate emission estimation techniques available as described below, unless such techniques are inappropriate. If such techniques are inappropriate, the Federal agency may obtain written approval from the appropriate EPA Regional Administrator for a modification or substitution, of another technique on a case-by-case basis or, where appropriate, on a generic basis for a specific Federal agency program. (1) For motor vehicle emissions, the most current version of the motor vehicle emissions model specified by EPA and available for use in the preparation or revision of SIPs in that State must be used for the conformity analysis as specified in paragraphs (b)(1)(i) and (ii) of this section:(i) The EPA must publish in the FEDERAL REGISTER a notice of availability of any new motor vehicle emissions model; and(ii) A grace period of 3 months shall apply during which the motor vehicle emissions model previously specified by EPA as the most current version may be used unless EPA announces a longer grace period in the FEDERAL REGISTER. Conformity analyses for which the analysis was begun during the grace period or no more than 3 months before the FEDERAL REGISTER notice of availability of the latest emission model may continue to use the previous version of the model specified by EPA.(2) For non-motor vehicle sources, including stationary and area source emissions, the latest emission factors specified by EPA in the "Compilation of Air Pollutant Emission Factors" (AP-42, http://www.epa.gov/ttn/chiefs/efpac) must be used for the conformity analysis unless more accurate emission data are available, such as actual stack test data from the stationary sources which are part of the conformity analysis.(c) The air quality modeling analyses required under this subpart must be based on the applicable air quality models, data bases, and other requirements specified in the most recent version of the "Guideline on Air Quality Models." (Appendix W to 40 CFR part 51 ). (1) The guideline techniques are inappropriate, in which case the model may be modified or another model substituted on a case-by-case basis or, where appropriate, on a generic basis for a specific Federal agency program; and(2) Written approval of the EPA Regional Administrator is obtained for any modification or substitution.(d) The analyses required under this subpart must be based on the total of direct and indirect emissions from the action and must reflect emission scenarios that are expected to occur under each of the following cases: (1) The attainment year specified in the SIP, or if the SIP does not specify an attainment year, the latest attainment year possible under the Act; or(2) The last year for which emissions are projected in the maintenance plan;(3) The year during which the total of direct and indirect emissions from the action is expected to be the greatest on an annual basis; and(4) Any year for which the applicable SIP specifies an emissions budget.58 FR 63253, Nov. 30, 1993, as amended at 75 FR 17277 , Apr. 5, 2010