(a) A commissioners court may not regulate under this subchapter farm machinery owned or operated by the person on whose property the machinery is located and kept on that property for purposes other than sale.
(b) A business subject to a screening requirement under Subchapter E of Chapter 391, Chapter 396, or Chapter 397, Transportation Code, that was in compliance with that screening requirement on August 26, 1991, is exempt from a screening requirement adopted under this subchapter.