25 C.F.R. § 170.454 – What design standards are used in the TTP?

(a) Depending on the nature of the project, Tribes must use appropriate design standards approved by FHWA. Appendix B to this subpart as well as the official Tribal Transportation Program Guide list the applicable design standards that can be used.
(b) All other design standards not listed in (a) must receive approval from FHWA.