Source: https://www.fhwa.dot.gov/fastact/factsheets/federalsharefs.cfm
Timestamp: 2019-04-19 22:20:33+00:00

Document:
The maximum share of project costs that may be funded with Federal-aid highway funds (the “Federal share”) varies based upon the Federal-aid program from which the project receives funding. In some cases the Federal share is also adjusted based on related statutory provisions. The FAST Act made relatively few changes to Federal share and those changes are shown in boldface below.
80% for any other project.
There are a number of exceptions and adjustments to the “standard” Federal share. See below (and program-specific fact sheets) for information about, and the applicability of, these exceptions and adjustments.
23 U.S.C. 120(a) and (b) authorize an upward adjustment (the “sliding scale”) to the Federal share for a State containing Federal and nontaxable Indian lands. A sliding scale State is eligible for an increased Federal share based upon the location of the project—90-95% for an applicable Interstate project and 80-95% for any other project. The amount of the upward adjustment is based on the percentage of Federal and nontaxable Indian lands in the State. (See FHWA guidance for the specific share allowable in each sliding scale State under this provision).
23 U.S.C. 120(h) authorizes a State to contribute an amount in excess of the non-Federal share of any project under title 23 U.S.C., thus decreasing the Federal share for the project.
1 The Federal share specified in this column may be subject (as specified) to one or more of the provisions referenced in the subsequent columns. In some circumstances the Federal share may also be modified by the provisions described above related to projects requiring Federal toll authority.
4 23 U.S.C. 120(c)(3), as amended by FAST Act § 1408(a).
6 MAP-21 § 1528(c), as amended by FAST Act § 1435. Increased Federal share applies through FY 2050.
13 23 U.S.C. 206(f) (which cites to 23 U.S.C. 120(b), with some specified exceptions).
16 FAST Act §1123(g). Federal funds other than those made available under titles 23 or 49 U.S.C. may be used for the non-Federal share.
19 23 U.S.C. 120(e)(2) as amended by FAST Act § 1408(b). Prior to the amendment, eligible Federal Lands Access Transportation Facilities qualified for a Federal share of 100%. Per § 421 of the Department of Transportation Appropriations Act, 2016 (P.L. 114-113 , the FAST Act amendment applies to projects to repair or reconstruct facilities damaged as a result of a qualifying natural disaster or catastrophic failure that occurs after October 1, 2015.
23 FAST Act §6004, 23 U.S.C. 503(c)(4)(J).

References: § 1408
 § 1528
 § 1435
 §1123
 § 1408
 § 421
 §6004