One of the reasons cited for putting up the walls is to protect the residents from the sound of the traffic. As this photo shows, there is a disconnect between the installation and official discourse.
A second reason for building walls along the side of the highway is for environmental protection. Maré has one of the poorest air quality ratings in the city as it sits between two major highways. The prevailing logic is to hide the problem behind an incomplete wall, but even if the wall were complete, particulate matter doesn't have much trouble hopping over a five meter barrier. A more thoughtful solution would be more trees which would absorb sound and CO2, while providing shade and also accomplish the city's goal of hiding the poverty from plain view.
And finally, to show that Cariocas are on top of the political scene here, some images from the city center. The first says "I don't think, I don't exist, I just watch" with the head in the shape of the OGlobo television network. The second is a reference to the Choque do Ordem "Shock Order" that is molding the city into a well-behaved, orderly little fellow.