Whenever a requirement is herein declared as to the distance from which certain lamps and devices shall render objects visible or within which such lamps or devices shall render objects visible, such provisions shall apply during the times stated in § 56-5-4450 in respect to a vehicle without load when upon a straight, level, unlighted highway under normal atmospheric conditions unless a different time or condition is expressly stated. Whenever a requirement is herein declared as to the mounted height of lamps or devices, it shall mean from the center of such lamp or device to the level ground upon which the vehicle stands when such vehicle is without load.