Iowa Code § 321.417: Single-beam road-lighting equipment.

Headlamps arranged to provide a single distribution of light not supplemented by auxiliary driving lamps shall be permitted on motor vehicles manufactured and sold prior to July 1, 1938, in lieu of multiple-beam road-lighting equipment specified in section 321.409 if the single distribution of light complies with the following requirements and limitations:

1. The headlamps shall be so aimed that when the vehicle is not loaded none of the high-intensity portion of the light shall at a distance of twenty-five feet ahead project higher than a level of five inches below the level of the center of the lamp from which it comes, and in no case higher than forty-two inches above the level on which the vehicle stands at a distance of seventy-five feet ahead.

2. The intensity of the light shall be sufficient to reveal persons and vehicles at a distance of at least two hundred feet.