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City Planner: Last year, the municipal transport authority converted one general-traffic lane into a dedicated express bus lane along each of the city's four main arterial roads. Over the subsequent year, average bus commute times dropped by 18 percent, while average automobile travel times on those arterial roads remained completely unchanged. Thus, converting general-traffic lanes to dedicated bus lanes does not reduce the volume of automobile traffic an arterial roadway can accommodate without congestion.

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

  1. A
    Automobile drivers using the arterial roads reported that overall traffic flow became more predictable after buses were restricted to dedicated lanes.
  2. B
    A municipal survey conducted prior to the lane conversion showed that a majority of local automobile commuters strongly opposed the reduction of general-traffic lanes.
  3. During the twelve-month period following the lane conversion, a substantial proportion of automobile commuters who previously used the arterial roads diverted their daily routes to adjacent residential streets.Answer
  4. D
    The automated traffic sensors used to record automobile travel times along the arterial roads were recalibrated once during the twelve-month monitoring period.
  5. E
    Whenever dedicated bus lanes are established in any metropolitan area, public transit passenger volume invariably increases by more than twenty percent.

Answer

The statement that many automobile commuters diverted their routes to adjacent residential streets most seriously weakens the argument.
The argument concludes that dedicated bus lanes do not diminish a roadway's automobile traffic capacity based on the evidence that automobile travel times did not increase. The correct choice weakens this claim by showing an alternate explanation for the constant travel times: a significant fraction of drivers diverted to residential side streets. Because fewer cars remained on the arterial roads, congestion did not increase, masking the loss of physical road capacity.

Step-by-Step Solution

1
Identify the premises and conclusion of the argument.
Premises: Converting one general lane to a bus lane decreased bus commute times by 18% and left car travel times unchanged on four arterial roads. Conclusion: Converting general lanes to bus lanes does not reduce the automobile traffic volume a roadway can handle without congestion.
Isolating the core reasoning reveals the gap between observed travel times and actual road capacity.
2
Identify the unstated assumption linking the premises to the conclusion.
The author assumes that the volume of automobile traffic attempting to use the arterial roads remained constant, meaning stable travel times must reflect unaltered road capacity.
Travel time is a function of both roadway capacity and total traffic volume.
3
Evaluate the answer choices for information that breaks this assumption.
If significant numbers of drivers avoided the arterial roads and diverted to residential streets, then the arterial roads handled fewer cars than before. This explains the unchanged travel times as a consequence of reduced demand rather than preserved capacity.
Providing an alternative explanation for the observed correlation directly severs the premise from the conclusion.

Key Concept

Alternative explanation for stable travel times (traffic displacement / spillover effect)
Estimated Time:1m 15s
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