14 C.F.R. § 121.93
(a) Each certificate holder conducting domestic or flag operations seeking a route approval must show—
(2) That the facilities and services required by §§ 121.97 through 121.107 are available and adequate for the proposed operation.
The Administrator approves a route outside of controlled airspace if he determines that traffic density is such that an adequate level of safety can be assured.
[Docket 6258, 29 FR 19194, Dec. 31, 1964, as amended by Amdt. 121-3, 30 FR 3638, Mar. 19, 1965; Amdt. 121-253, 61 FR 2610, Jan. 26, 1996]