40 CFR § 93.110 - Criteria and procedures: Latest planning assumptions.
(a) Except as provided in this paragraph, the conformity determination, with respect to all other applicable criteria in §§ 93.111 through 93.119, must be based upon the most recent planning assumptions in force at the time the conformity analysis begins. The conformity determination must satisfy the requirements of paragraphs (b) through (f) of this section using the planning assumptions available at the time the conformity analysis begins as determined through the interagency consultation process required in § 93.105(c)(1)(i). The “time the conformity analysis begins” for a transportation plan or TIP determination is the point at which the MPO or other designated agency begins to model the impact of the proposed transportation plan or TIP on travel and/or emissions. New data that becomes available after an analysis begins is required to be used in the conformity determination only if a significant delay in the analysis has occurred, as determined through interagency consultation.
(b) Assumptions must be derived from the estimates of current and future population, employment, travel, and congestion most recently developed by the MPO or other agency authorized to make such estimates and approved by the MPO. The conformity determination must also be based on the latest assumptions about current and future background concentrations.
(c) The conformity determination for each transportation plan and TIP must discuss how transit operating policies (including fares and service levels) and assumed transit ridership have changed since the previous conformity determination.
(f) Key assumptions shall be specified and included in the draft documents and supporting materials used for the interagency and public consultation required by § 93.105.