Whenever the state traffic commission shall determine that any prima facie limit set forth in § 31-14-2 is greater than is reasonable in any historic district listed on the National Register of Historic Places or on any road duly recognized as a scenic highway by the scenic highway board in accordance with the provisions of chapter 14 of Title 24, the commission shall determine and declare a reasonable and safe prima facie limit, in the manner set forth in § 31-14-4, in order to provide for the preservation of the historical character of the district or the scenic quality of the highway, provided that:

(1)  The reduced prima facie limit shall be for a length of road not to exceed one mile or the distance between the boundaries of the district, whichever distance is the lesser, and

(2)  The reduced prima facie limit shall not result in traffic congestion.