Iowa Code § 321.299: Overtaking a vehicle.

The following rules shall govern the overtaking and passing of vehicles proceeding in the same direction, subject to those limitations, exceptions, and special rules hereinafter stated:

1. The driver of a vehicle overtaking another vehicle proceeding in the same direction shall pass to the left of the other vehicle at a safe distance and shall not again drive to the right side of the roadway until safely clear of the overtaken vehicle.

2. Except when overtaking and passing on the right is permitted, the driver of an overtaken vehicle shall give way to the right in favor of the overtaking vehicle and shall not increase the speed of the overtaken vehicle until completely passed by the overtaking vehicle.