Tex. Transportation Code § 370.034

(a) If an authority determines that the traffic needs of the counties in which it operates and the traffic needs of the surrounding region could be most efficiently and economically met by jointly operating two or more transportation projects as one operational and financial enterprise, it may create a system made up of those transportation projects. An authority may create more than one system and may combine two or more systems into one system. An authority may finance, acquire, construct, and operate additional transportation projects as additions to or expansions of a system if the authority determines that the transportation project could most efficiently and economically be acquired or constructed if it were a part of the system and that the addition will benefit the system.

(b) The revenue of a system shall be accounted for separately and may not be commingled with the revenue of a transportation project that is not a part of the system or with the revenue of another system.