@rmkrutz@crimson.ua.edu The main problem with (D) is that it deals with the wrong period of time. (D) talks about the period of time while the highway was being built, and the question is talking about after the highway has been opened, which makes (D) irrelevant. The original statement also says "the average commute time for workers IN downtown Gastner increase." It says nothing about the commute time from the suburbs.
Therefore, (E) provides the best explanation. It makes sense that a highway that links several suburbs to the downtown area would increase the concentration of vehicles immediately adjacent to the highway, thus overtaxing the connecting roads and slowing the flow of traffic on the streets around the highway.