Iowa Code § 321.420: Number of lamps lighted.

Whenever a motor vehicle equipped with headlamps as required in this chapter is also equipped with any auxiliary lamps or a spot lamp or any other lamp on the front of the vehicle, that projects a beam of an intensity greater than three hundred candlepower, not more than a total of four of any such lamps on the front of the vehicle shall be lighted at any one time when upon a highway.