content: I'm guessing that the decision was made on the basis of cost and time.  Constructing a roof to me sounds extremely expensive; such a construction would require 1) the materials, 2) the labor, 3) possible maintenance (maybe limited, depending on structural design).  Road surface maintenance is already scheduled annually, so from a quick layman's judgment you'd think they were trying to kill two birds with one stone, with the hopes that the installation could coincide with road repair efforts.