; (2) how volunteer protections in subsection (f)(2) were complied with; and (3) whether per-mile user fees can maintain the long-term solvency of the Highway Trust Fund and achieve and maintain a state of good repair in the surface transportation system. (m) Sunset The pilot program established under this section shall expire on the date that is 4 years after the date on which volunteer participants begin participating in such program. (n) Definitions In this section, the following definitions apply: (1) Commercial vehicle The term commercial vehicle has the meaning given the term commercial motor vehicle in section 31101 of title 49, United States Code. (2) Highway trust fund The term Highway Trust Fund means the Highway Trust Fund established under section 9503 of the Internal Revenue Code of 1986. (3) Light truck The term light truck has the meaning given the term in section 523.2 of title 49, Code of Federal Regulations. (4) Medium- and heavy-duty truck The term medium- and heavy-duty truck has the meaning given the term commercial medium- and heavy-duty on-highway vehicle in section 32901(a) of title 49, United States Code. (5) Per-mile user fee The term per-mile user fee means a revenue mechanism that— (A) is applied to road users operating motor vehicles on the surface transportation system; and (B) is based on the number of vehicle miles traveled by an individual road user. (6) Volunteer participant The term volunteer participant means— (A) an owner or lessee of an individual private motor vehicle who volunteers to participate in the pilot program; (B) a commercial vehicle operator who volunteers to participate in the pilot program