Iowa Code § 321.233
This chapter, except sections 321.277 and 321.280, does not apply to persons and motor vehicles and other equipment while actually engaged in work upon the surface of a highway officially closed to traffic but does apply to such persons and vehicles when traveling to or from such work. The minimum speed restriction of section 321.285, subsection 5, and the provisions of sections 321.297, 321.298, and 321.323 do not apply to road workers operating maintenance equipment on behalf of any state or local authority while engaged in road maintenance, road blading, snow and ice control and removal, and granular resurfacing work on a highway, whether or not the highway is closed to traffic.
Iowa Code § 321.233
85 Acts, ch 167, §1; 86 Acts, ch 1220, §36; 90 Acts, ch 1230, §64; 97 Acts, ch 104, §18; 2009 Acts, ch 133, § 239