Iowa Code Title VIII. Transportation [Chs. 306-330B] § 321.415. Required usage of lighting devices

a. Whenever a driver of a vehicle approaches an oncoming vehicle within one thousand feet, the driver shall use a distribution of light, or composite beam, so aimed that the glaring rays are not projected into the eyes of the oncoming driver. The lowermost distribution of light, or composite beam, specified in section 321.409, subsection 1, paragraph “b”, shall be deemed to avoid glare at all times, regardless of road contour and loading.

b. Whenever the driver of a vehicle follows another vehicle within four hundred feet to the rear, except when engaged in the act of overtaking and passing, the driver shall use a distribution of light permissible under this chapter other than the uppermost distribution of light specified in section 321.409, subsection 1, paragraph “a”.