(2)Except for amounts identified in section 403(f), funds described in paragraph (1) shall be apportioned 75 per centum in the ratio which the population of each State bears to the total population of all the States, as shown by the latest available Federal census, and 25 per centum in the ratio which the public road mileage in each State bears to the total public road mileage in all States. For the purposes of this subsection, a “public road” means any road under the jurisdiction of and maintained by a public authority and open to public travel. Public road mileage as used in this subsection shall be determined as of the end of the calendar year preceding the year in which the funds are apportioned and shall be certified to by the Governor of the State and subject to approval by the Secretary. The annual apportionment to each State shall not be less than three-quarters of 1 percent of the total apportionment, except that the apportionment to the Secretary of the Interior shall not be less than 2 percent of the total apportionment and the apportionments to the Virgin Islands, Guam, American Samoa, and the Commonwealth of the Northern Mariana Islands shall not be less than one-quarter of 1 per centum of the total apportionment. A highway safety program approved by the Secretary shall not include any requirement that a State implement such a program by adopting or enforcing any law, rule, or regulation based on a guideline promulgated by the Secretary under this section requiring any motorcycle operator eighteen years of age or older or passenger eighteen years of age or older to wear a safety helmet when operating or riding a motorcycle on the streets and highways of that State. Implementation of a highway safety program under this section shall not be construed to require the Secretary to require compliance with every uniform guideline, or with every element of every uniform guideline, in every State. A State may use the funds apportioned under this section, in cooperation with neighboring States, for highway safety programs or related projects that may confer benefits on such neighboring States. Funds apportioned under this section to any State, that does not have a highway safety program approved by the Secretary or that is not implementing an approved program, shall be reduced by amounts equal to not less than 20 percent of the amounts that would otherwise be apportioned to the State under this section, until such time as the Secretary approves such program or determines that the State is implementing an approved program, as appropriate. The Secretary shall consider the gravity of the State’s failure to have or implement an approved program in determining the amount of the reduction.