The following political subdivisions and other public entities may become a part of a district with the approval of the governing body of the political subdivision or public entity:
(1) a county located adjacent to the county in which a creating municipality is located;
(2) a municipality with a population of more than 18,000 located in a county described by Subdivision (1); and
(3) a public entity located in a county that has become part of the district.
Official text of the Texas Transportation Code as published by the Texas Legislative Council.