23 C.F.R. § 1240 app D to Part 1240
A. To determine the national average seat belt use rate in a calendar year, each State seat belt use rate for the calendar year will be weighted to reflect the percentage of total national vehicle miles traveled attributable to that State.
B. If a State seat belt use rate is unavailable for a State during a calendar year (either because the State did not conduct a seat belt use survey or a survey was conducted but does not comply with the Uniform Criteria for State Observational Surveys of Seat Belt Use, 23 CFR Part 1340 ), NHTSA will calculate a State seat belt use rate, using the last available State seat belt use rate determined under § 1240.11 or § 1240.12 of this part, as applicable, along with information on seat belt use rates from the FARS, and an algorithm relating FARS seat belt use rates to observed seat belt use rates (see Appendix 1, note). This procedure will produce an estimated State seat belt use rate for the unknown calendar year. The estimated State seat belt use rate will then be weighted in the manner described in paragraph A of this appendix.
C. The national average seat belt use rate for the calendar year will be determined by adding the weighted State seat belt use rates for each of the States (i.e., the national average seat belt use rate is the weighted average of all the State seat belt use rates).
D. NHTSA may elect to use a seat belt use survey that does not comply with the Uniform Criteria for State Observational Surveys of Seat Belt Use in determining the national average seat belt use rate (even though the State that submitted the survey is ineligible to receive an allocation of funds), if in NHTSA's judgment, the deficiencies in the survey are not so substantial as to render the survey less accurate than the FARS estimate.
23 C.F.R. § 1240 app D to Part 1240