Tex. Const. art. III, § 49-p

(a) To provide funding for highway improvement projects, the legislature by general law may authorize the Texas Transportation Commission or its successor to issue general obligation bonds of the State of Texas in an aggregate amount not to exceed $5 billion and enter into related credit agreements. The bonds shall be executed in the form, on the terms, and in the denominations, bear interest, and be issued in installments as prescribed by the Texas Transportation Commission or its successor.

(b) A portion of the proceeds from the sale of the bonds and a portion of the interest earned on the bonds may be used to pay:

(1) the costs of administering projects authorized under this section;

(2) the cost or expense of the issuance of the bonds; and

(3) all or part of a payment owed or to be owed under a credit agreement.

(c) The bonds authorized under this section constitute a general obligation of the state. While any of the bonds or interest on the bonds is outstanding and unpaid, there is appropriated out of the first money coming into the treasury each fiscal year, not otherwise appropriated by this constitution, an amount sufficient to pay the principal of and interest on the bonds that mature or become due during the fiscal year, including an amount sufficient to make payments under a related credit agreement.

(d) Bonds issued under this section, after approval by the attorney general, registration by the comptroller of public accounts, and delivery to the purchasers, are incontestable and are general obligations of the State of Texas under this constitution.