It is widely accepted that a single noise barrier beside a highway will provide a degree of noise reduction for neighboring homes. An interesting issue arises when residential areas line both sides of the highway, requiring two parallel barriers, one on each side of the highway. An abundance of research has been carried out to examine this situation. The research suggests that this configuration allows for multiple reflections to occur between the two barriers, unfortunately resulting in less noise reduction for both residential areas. Researchers attacking this problem have uncovered many relevant issues. This paper will summarize these issues and will class them into those in which there is consensus and those in which there is no consensus. Researchers agree that multiple reflections exist in a parallel wall situation, that these reflections cause an insertion loss degradation, that the insertion loss degradation can be controlled, and that making barriers absorptive or tilting them is effective in reducing the degradation. They do not agree on the extent of insertion loss degradation, the significance of environmental variables such as ground cover, or the limits to improvements from absorptive or tilting barriers.