(a) The commissioners court of a county, for a county road or highway outside the limits of the right-of-way of an officially designated or marked highway or road of the state highway system and outside a municipality, has the same authority to increase prima facie speed limits from the results of an engineering and traffic investigation as the Texas Transportation Commission on an officially designated or marked highway of the state highway system.
(b) The commissioners court of a county may declare a lower speed limit of not less than:
(1) 30 miles per hour on a county road or highway to which this section applies, if the commissioners court determines that the prima facie speed limit on the road or highway is unreasonable or unsafe; or
(2) 20 miles per hour:
(A) in a residence district, unless the roadway has been designated as a major thoroughfare by a city planning commission; or
(B) on a county road or highway to which this section applies that is located within 500 feet of an elementary, secondary, or open-enrollment charter school or an institution of higher education, if approved under § 545.357.

(c) The commissioners court may not modify the rule established by § 545.351(a) or establish a speed limit of more than 70 miles per hour.
(d) The commissioners court may modify a prima facie speed limit in accordance with this section only by an order entered on its records.
(e) The commissioners court of a county with a population of more than 2.8 million may establish from the results of an engineering and traffic investigation a speed limit of not more than 75 miles per hour on any part of a highway of that county that is a limited-access or controlled-access highway, regardless of the location of the part of the highway.