Tex. Transportation Code § 547.301: GENERAL PROVISIONS RELATING TO MEASUREMENTS.

(a) Unless expressly stated otherwise, a visibility distance requirement imposed by this chapter for a lamp or device applies when a lighted lamp or device is required and is measured as if the vehicle were unloaded and on a straight, level, unlighted highway under normal atmospheric conditions.

(b) A mounted height requirement imposed by this chapter for a lamp or device is measured as if the vehicle were unloaded and on level ground and is measured from the center of the lamp or device to the ground.

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