Hillsburgh and Nansensville are two major metropolitan areas only 50 m

Official Explanation

The credited answer is (B). If the train connecting the cities was no longer available, due to a strike, many more people would have to take cars, which would lead to more traffic. Even though the Metro Expressway opened, the increase in traffic due to the train strike could lead to continued congestion.

If the Metro Expressway opened when most people leave on vacation, then we would expect the traffic to be even less than usual. Even if traffic temporarily increased as people left, this would not cause increased congestion for an entire month, and if some of the commuters are away on vacation then there would be less traffic than normal on a daily basis. This certainly would not explain why the traffic would remain at a high level of congestion. Choice (A) is incorrect.

The composition of the road may have consequences several years down the line, when one of the two road may need renovation & repairs before the other does, but it doesn't explain, right now, why the new road didn't relieve traffic congestion. Choice (C) is incorrect.

If police activity decreased on the Interstate Highway, then perhaps some portion of the folks on the road feel at liberty to drive faster than the speed limit, but the influence on congestion is less clear. If the police cars split up between the two road, why wouldn't all the cars do that, resulting in less congestion overall? Choice (D) is incorrect.

If there's another expressway in the future, that might affect future traffic patterns, but that doesn't do anything to explain why, right now, there's still a great deal of traffic congestion. Choice (E) is incorrect.