Iowa Code § 314.24
Cities, counties, and the department shall to the extent practicable preserve and protect the natural and historic heritage of the state in the design, construction, reconstruction, relocation, repair, or maintenance of roads, streets, or highways. Destruction or damage to natural areas, including but not limited to prime agricultural land, parks, preserves, woodlands, wetlands, recreation areas, greenbelts, historical sites, or archaeological sites shall be avoided, if reasonable alternatives are available for the location of roads, streets, or highways at no significantly greater cost. In implementing this section, cities, counties, and the department shall make a diligent effort to identify and examine the comparative cost of utilizing alternative locations for roads, streets, or highways.
Iowa Code § 314.24