Iowa Code § 321.303: Limitations on overtaking on the left.

A vehicle shall not be driven to the left side of the center of the roadway in overtaking and passing another vehicle proceeding in the same direction unless the left side is clearly visible and is free of oncoming traffic for a sufficient distance ahead to permit the overtaking and passing to be completely made without interfering with the safe operation of a vehicle approaching from the opposite direction or a vehicle overtaken. The overtaking vehicle shall return to the right-hand side of the roadway before coming within three hundred feet of a vehicle approaching from the opposite direction when traveling on a roadway having a legal speed limit in excess of thirty miles per hour, and the overtaking vehicle shall return to the right-hand side of the roadway before coming within one hundred feet of a vehicle approaching from the opposite direction when traveling on a roadway having a legal speed limit of thirty miles per hour or less.