25 C.F.R. § 170.627
In addition to all funds made available under chapter 2 of title 23, the cooperation of States, counties, and other local subdivisions may be accepted in construction and improvement of a Tribal transportation facility. In accordance with 23 U.S.C. 202(a)(9) , any funds received from a State, county, or local subdivision may be credited by the Secretaries to appropriations available for the TTP. Subject to an agreement among the Tribe, BIA or FHWA, and the State, county, or local subdivision that addresses the purpose and intent of the funds such funds may be provided to the Tribe through an a self-determination contract, self-governance agreement, Program Agreement or other appropriate agreement developed in accordance with 23 U.S.C. 202(b)(6) & (b)(7).
25 C.F.R. §170.627