Tex. Transportation Code § 366.031

(a) Two or more counties, acting through their respective commissioners courts, may by order passed by each commissioners court create a regional tollway authority under this chapter if:

(1) one of the counties has a population of not less than 300,000;

(2) the counties form a contiguous territory; and

(3) unless one of the counties has a population of 2.5 million or more, the commission approves the creation.

(b) The commission shall adopt rules to implement the provisions of this section by March 1, 1998.

(c) A commissioners court may by resolution petition an established authority for inclusion in the authority if the county is contiguous to a county that initially created the authority.

(d) On approval of the board of an authority receiving a petition under Subsection (c), the county becomes part of the authority.

(e) A county that is not part of an authority and in which an authority turnpike project is located becomes part of the authority on the date the authority determines that:

(1) recorded electronic toll collections at toll assessment facilities located in the county account for not less than four percent of all recorded electronic toll collections on all of the authority's turnpike projects; and

(2) the population of the county is at least four percent of the aggregate population of all the counties of the authority, not including the county that will become part of the authority.