(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.