Iowa Code § 321.302: Overtaking and passing.

1. Unless otherwise prohibited by law, the driver of a vehicle on a roadway with unobstructed pavement of sufficient width for two or more lines of traffic moving in the same direction as the vehicle being passed may overtake and pass upon the right of another vehicle which is making or about to make a left turn when such movement can be made in safety.

2. Unless otherwise prohibited by law, the driver of a vehicle may overtake and, allowing sufficient clearance, pass another vehicle proceeding in the same direction either upon the left or upon the right on a roadway with unobstructed pavement of sufficient width for four or more lines of moving traffic when such movement can be made in safety.

3. The driver of a vehicle shall not drive off the pavement or upon the shoulder of the roadway or upon the apron or roadway of an intersecting roadway in overtaking or passing on the right or the left.

4. A person convicted of a violation of this section is guilty of a simple misdemeanor punishable as a scheduled violation under section 805.8A, subsection 6.