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Question

Difficulty: MediumNecessary Assumption

Urban Planner: To decrease traffic congestion in the downtown commercial district, the city plans to implement a new daily toll for any private vehicle entering the district during peak commuting hours. The city anticipates that this policy will successfully reduce the number of private vehicles entering the district. Clearly, this toll will achieve its goal of significantly reducing overall traffic congestion in the downtown commercial district.

Which one of the following is an assumption on which the urban planner's argument depends?

  1. A
    The daily toll will cause most of the people who currently commute in private vehicles to switch to using public transportation.
  2. B
    Private vehicles are responsible for a larger share of the downtown traffic congestion than all other types of vehicles combined.
  3. The anticipated decrease in the number of private vehicles will not be accompanied by a corresponding increase in the number of commercial or public vehicles entering the downtown commercial district.Answer
  4. D
    Other municipalities that have instituted tolls on private vehicles have successfully reduced overall traffic congestion in their commercial districts.
  5. E
    If the city does not implement the proposed toll, the volume of private vehicles entering the downtown commercial district will continue to grow.

Answer

The argument depends on the assumption that the anticipated decrease in private vehicles will not be offset by a corresponding increase in other types of vehicles.
This statement is logically required by the argument. The argument concludes that overall traffic congestion will decrease based solely on the premise that private vehicle traffic will decrease. For this conclusion to hold, it must be true that other forms of traffic (like commercial or public vehicles) do not increase enough to offset the reduction in private vehicles. If they did, overall congestion would not decrease.

Step-by-Step Solution

1
Identify the main conclusion of the argument.
The conclusion is that the daily toll will significantly reduce overall traffic congestion in the downtown commercial district.
Understanding the exact scope of the conclusion is the first step in evaluating any logical argument.
2
Identify the supporting premise.
The premise is that the toll policy will successfully reduce the number of private vehicles entering the district.
We must determine what evidence the author uses to justify the conclusion.
3
Find the logical gap between the premise and the conclusion.
The argument shifts from a reduction in one specific type of vehicle (private) to a reduction in 'overall' traffic congestion. This ignores the possibility that other types of traffic might fill the void.
Necessary assumptions often bridge a gap between the specific terms in the premises and the broader terms in the conclusion.
4
Apply the Negation Test to the suspected necessary assumption.
Negating the correct statement yields: 'The decrease in private vehicles WILL be accompanied by a corresponding increase in commercial or public vehicles.' This negated statement destroys the conclusion because overall traffic would not decrease.
The Negation Test is the definitive way to prove whether a statement is strictly necessary for an argument to logically hold.

Key Concept

Necessary assumptions must be logically required for the conclusion to be true; often they defend the argument against alternative possibilities that would prevent the premise from leading to the conclusion.
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Topics

Necessary Assumption

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