Tex. Const. art. III, § 49-n (H.J.R. 28, 2003)

(a) To fund highway improvement projects, the legislature may authorize the Texas Transportation Commission or its successor to issue bonds and other public securities and enter into bond enhancement agreements that are payable from revenue deposited to the credit of the state highway fund.

(b) In each fiscal year in which amounts become due under the bonds, other public securities, or agreements authorized by this section, there is appropriated from the revenue deposited to the credit of the state highway fund in that fiscal year an amount that is sufficient to pay:

(1) the principal of and interest on the bonds or other public securities that mature or become due during the fiscal year; and

(2) any cost related to the bonds and other public securities, including payments under bond enhancement agreements, that becomes due during that fiscal year.

(c) Any dedication or appropriation of revenue to the credit of the state highway fund may not be modified so as to impair any outstanding bonds or other public securities secured by a pledge of that revenue unless provisions have been made for a full discharge of those securities.