Whenever a red reflector is required or permitted to be used in substitution of lamps upon a vehicle under any one of the provisions of this chapter, such reflector shall be mounted upon the vehicle at a height not to exceed forty-two inches nor less than twenty inches above the ground upon which the vehicle stands, and every such reflector shall be so designed and maintained as to be visible at night from all distances within three hundred feet to fifty feet from such vehicle, except that on a commercial vehicle the reflector shall be visible from all distances within five hundred feet to fifty feet from such vehicle when directly in front of a motor vehicle displaying lawfully lighted headlamps as provided in section 321.409.