Texas Transportation Code - TRANSP § 457.302. Maximum Tax Rate

(1) the voters of the authority approve the authority's sales and use tax rate or rate increase at an election held on the same day on which the municipality or county having territory in the jurisdiction of the authority adopts a sales and use tax or an additional sales and use tax; and

(2) the combined rates of all sales and use taxes imposed by the authority and all political subdivisions of this state would exceed two percent in any part of the territory in the jurisdiction of the authority.