5 U.S.C. § 5731

(a) The allowance for actual expenses for transportation may not exceed the lowest first-class rate by the transportation facility used unless it is certified, in accordance with regulations prescribed under section 5738 of this title , that— (1) lowest first-class accommodations are not available; or (2) use of a compartment or other accommodation authorized or approved by the head of the agency concerned or his designee is required for security purposes. (b) Instead of the maximum fixed by subsection (a) of this section, the allowance to an employee of the Government for actual expenses for transportation on an inter-island steamship in Hawaii may not exceed the rate for accommodations on the steamship that is equivalent as nearly as possible to the rate for the lowest first-class accommodations on trans-pacific steamships.