Iowa Code § 321.348: Limitations on cities.

It shall be unlawful for any city to close or obstruct any street or highway which is used as the extension of a primary road within such city, except at times of fires or for the purpose of doing construction or repair work on such street or highway, or for other reasons with the consent of the department, and it shall also be unlawful for any city to erect or cause to be erected or maintained any traffic sign or signal inconsistent with the provisions of this chapter.