Iowa Code § 321.304: Prohibited passing.

No vehicle shall, in overtaking and passing another vehicle or at any other time, be driven to the left side of the roadway under the following conditions:

1. When approaching the crest of a grade or upon a curve in the highway where the driver’s view along the highway is obstructed for a distance of approximately seven hundred feet.

2. When approaching within one hundred feet of any narrow bridge, viaduct, or tunnel, when so signposted, or when approaching within one hundred feet of or traversing any intersection or railroad grade crossing.

3. Where official signs are in place directing that traffic keep to the right or a distinctive center line or off-center line is marked, which distinctive line also so directs traffic as declared in the sign manual adopted by the department of transportation.