Tex. Transportation Code § 545.363: MINIMUM SPEED REGULATIONS.

(a) An operator may not drive so slowly as to impede the normal and reasonable movement of traffic, except when reduced speed is necessary for safe operation or in compliance with law.

(b) When the Texas Transportation Commission, the Texas Turnpike Authority, the commissioners court of a county, or the governing body of a municipality, within the jurisdiction of each, as applicable, as specified in Sections 545.353 to 545.357, determines from the results of an engineering and traffic investigation that slow speeds on a part of a highway consistently impede the normal and reasonable movement of traffic, the commission, authority, county commissioners court, or governing body may determine and declare a minimum speed limit on the highway.

(c) If appropriate signs are erected giving notice of a minimum speed limit adopted under this section, an operator may not drive a vehicle more slowly than that limit except as necessary for safe operation or in compliance with law.

Acts 1995, 74th Leg., ch. 165, Sec. 1, eff. Sept. 1, 1995.