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City Council Member: Last month, we installed a new, synchronized traffic light system at the Oak Street intersection. This month, the number of traffic accidents at that intersection dropped by 30% compared to the previous month. Therefore, the new traffic light system has clearly improved safety by reducing accidents at the Oak Street intersection.

Which of the following, if true, most seriously weakens the city council member's argument?

  1. During the same month, major roadwork closed the primary highway feeding into the Oak Street intersection, drastically reducing the volume of traffic passing through it.Answer
  2. B
    Other intersections in the city that recently received identical traffic light systems also reported significant drops in accident rates.
  3. C
    The council member's claim that accidents dropped by 30% is slightly inaccurate; the official police report indicates the decrease was exactly 28%.
  4. D
    Although the overall percentage of accidents decreased, the average financial cost of the property damage for the remaining accidents increased.
  5. E
    A survey conducted last week showed that local drivers feel less stressed when driving through intersections equipped with synchronized lights.

Answer

The statement regarding major roadwork closing the primary highway and reducing traffic volume.
The argument relies on a flawed causal assumption: because the drop in accidents followed the installation of the traffic lights, the lights must have caused the drop. To weaken a causal argument, you should look for an alternate cause for the observed effect. The correct answer provides a perfect alternate cause: if major roadwork prevented cars from entering the intersection, the number of accidents would naturally decrease, completely independently of the new traffic light system.

Step-by-Step Solution

1
Identify the conclusion of the argument.
The conclusion is that the new traffic light system has improved safety by reducing accidents at the Oak Street intersection.
Understanding what the author is trying to prove is the first step in evaluating any argument.
2
Identify the premises supporting the conclusion.
The premise is that a new traffic light system was installed last month, and this month, accidents at that intersection dropped by 30%.
Premises represent the evidence used to justify the conclusion.
3
Analyze the logical gap between the premises and the conclusion.
The argument assumes a causal relationship based merely on a sequence of events. It assumes no other factors could have caused the 30% drop in accidents.
Finding the assumption or logical flaw reveals how the argument can be weakened.
4
Evaluate the options to find one that exploits this gap.
The option about major roadwork reducing traffic volume provides an alternate cause for the drop in accidents.
An alternate cause severely weakens a causal conclusion by showing the stated effect could have occurred without the stated cause.

Key Concept

Evaluating Causal Arguments and Identifying Alternate Explanations
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