a) The analyses required under this Part must be based on the latest planning assumptions.

1. 1) All planning assumptions must be derived from the estimates of population, employment, travel, and congestion most recently approved by the MPO, regional planning commission or other agency authorized to make such estimates, where available.
2. 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, shall be developed in consultation with the regional planning commission, MPO or other agency authorized to make such estimates for the urban area.

b) The analyses required under this Part 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 and written approval of the USEPA Regional Administrator is obtained for any modification or substitution, they may be modified or another technique substituted on a case-by-case basis or, where appropriate, on a generic basis for a specific Federal agency program. Any modification and substitution shall be done in consultation with IEPA and USEPA.

1. 1) For motor vehicle emissions, the most current version of the motor vehicle emissions model specified by USEPA and available for use in the preparation or revision of SIPs in this State must be used for the conformity analysis as specified in subsections (b)(1)(A) and (B) of this Section:

   1. A) The USEPA must publish in the Federal Register a notice of availability of any new motor vehicle emissions model; and
   2. B) A grace period of 3 months shall apply during which the motor vehicle emissions model previously specified by USEPA as the most current version may be used. Conformity analyses for which the analysis was begun during the grace period or no more than 3 years before the Federal Register notice of availability of the latest emission model may continue to use the previous version of the model specified by USEPA.
2. 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)" as incorporated by reference in Section 255.160 of this Part must be used for the conformity analysis unless more accurate emission data are available, such as actual stack test data from stationary sources which are part of the conformity analysis.

c) Any air quality modeling analyses necessary to demonstrate conformity shall 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" Part 51 – Requirements for Preparation, Adoption, and Submittal of Implementation Plans, 40 CFR 51, Appendix W, 61 Fed. Reg. 41837 (August 12, 1996) incorporated by reference in Section 255.160 of this Part, unless:

1. 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. 2) Any modification of substitution shall be done in consultation with USEPA.

d) The analyses required under this Part, except Section 255.220(a)(1) of this Part, 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. 1) The CAA mandated attainment year or, if applicable, the farthest year for which emissions are projected in the maintenance plan;
2. 2) 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
3. 3) Any year for which the applicable SIP specifies an emissions budget.