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Difficulty: MediumFlaw in the Reasoning

City Planner: Every municipality that successfully reduces traffic congestion in its downtown core invests significantly in expanding light rail infrastructure. Riverdale recently completed a multi-million-dollar expansion of its downtown light rail network. Therefore, Riverdale will undoubtedly experience a significant reduction in downtown traffic congestion over the coming year.

Which of the following most accurately describes a flaw in the city planner's reasoning?

  1. A
    presumes without justification that light rail expansion is the only method available to reduce downtown traffic
  2. B
    calls into question whether the municipality actually completed the expansion of its light rail network
  3. infers that a condition required for a desired outcome is sufficient to ensure that the outcome will occurAnswer
  4. D
    bases a broad generalization about all municipalities on the single case of Riverdale
  5. E
    confuses an intermediate conclusion supporting the plan with the main conclusion regarding traffic levels

Answer

The argument is flawed because it treats a necessary condition for reducing congestion as though it were sufficient to guarantee that reduction.
The planner states that every municipality that reduces traffic invests in light rail, meaning light rail investment is a necessary condition for congestion reduction. However, the planner concludes that because Riverdale invested in light rail, congestion reduction is guaranteed. This erroneously treats a necessary condition as a sufficient condition.

Step-by-Step Solution

1
Analyze the premises and formalize the conditional relationship.
Premise 1 states: If a municipality successfully reduces congestion, then it invests in light rail (Reduced Congestion → Light Rail Investment). Premise 2 states: Riverdale has invested in light rail.
Identifying the formal relationship between the terms reveals the structural logic of the argument.
2
Evaluate the relationship between the premises and the conclusion.
The conclusion claims that Riverdale will definitely reduce traffic congestion (Light Rail Investment → Reduced Congestion).
Comparing the direction of the premise conditional with the conclusion reveals whether the deduction is valid or commits a formal fallacy.
3
Identify the primary reasoning vulnerability.
The author commits the fallacy of affirming the consequent (or confusing necessary and sufficient conditions) by treating a requirement as a guarantee.
Matching the structural gap to standard flaw descriptions pinpoints the correct answer.

Key Concept

Mistaking a Necessary Condition for a Sufficient Condition (Conditional Reversal)
Estimated Time:1m 15s
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Topics

Flaw in the Reasoning

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