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Urban Planner: To reduce traffic congestion, the city fully subsidized public transit passes for all car-owning residents of the central business district. Six months later, average morning rush-hour commute times for vehicles entering the central business district have decreased by twenty percent. This clearly demonstrates that the transit subsidy successfully motivated a large number of these residents to switch from driving to taking public transit.

Which one of the following, if true, most seriously weakens the urban planner's argument?

  1. A
    Over the last six months, ridership on public transit routes that transport commuters from the suburbs into the central business district has slightly declined.
  2. B
    A follow-up survey revealed that most central business district residents who utilized the subsidy did so to save money on daily parking fees rather than to shorten their commute times.
  3. The vast majority of employed car-owning residents of the central business district commute to workplaces located entirely outside the district.Answer
  4. D
    The sensors used by the city to calculate average commute times are calibrated to ignore commercial delivery trucks, which make up a significant portion of morning traffic.
  5. E
    The twenty percent decrease in morning rush-hour commute times was matched by an identical twenty percent decrease in average commute times during the evening rush hour.

Answer

The statement regarding residents commuting to workplaces located entirely outside the district most seriously weakens the argument.
The correct answer severs the causal relationship by highlighting a directional scope shift. The premise cites a decrease in vehicles entering the district during the morning. However, if the subsidized residents work outside the district, their morning commute would be exiting the district. Therefore, a change in their driving habits could not possibly explain the decrease in entering traffic, destroying the author's conclusion that the subsidy caused the observed improvement.

Step-by-Step Solution

1
Identify the conclusion of the argument.
The transit subsidy motivated a large number of central business district (CBD) residents to switch from driving to taking public transit.
This is the author's primary claim, indicated by the phrase 'This clearly demonstrates that...'
2
Identify the premise supporting the conclusion.
Average morning rush-hour commute times for vehicles entering the CBD have decreased by 20%.
The author uses this single data point as the sole evidence for the conclusion.
3
Analyze the logical gap (assumption) between the premise and the conclusion.
The argument assumes that a reduction in driving by CBD residents would cause a decrease in traffic entering the CBD during the morning.
For the conclusion to be valid based on the premise, the group receiving the subsidy (residents) must be the same group responsible for the specific traffic metric that improved (morning inbound traffic).
4
Evaluate the options to find the one that attacks this assumption.
The option stating that residents commute to workplaces outside the district attacks the assumption.
If residents work outside the CBD, their morning commute is outbound. If they stop driving, it reduces outbound traffic, not inbound traffic. Therefore, the subsidy cannot be the cause of the decreased inbound commute times.

Key Concept

Identifying Causation vs. Correlation and Scope Shifts (Directionality)
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