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Difficulty: MediumResolve the Paradox

Five years ago, the municipal transit agency in the city of Vaelen replaced all diesel buses on its primary express corridors with electric buses equipped with substantially quieter electric motors. Urban planners anticipated that eliminating diesel engine noise would significantly reduce peak-hour ambient roadside noise levels along these corridors. However, acoustic monitoring conducted over the four years following the transition revealed that average peak-hour roadside noise along these routes actually increased slightly, despite traffic volumes, vehicle speeds, and pavement conditions remaining unchanged. Which of the following, if true, most helps to resolve the apparent discrepancy described above?

  1. A
    The diesel buses previously operated on the express corridors produced intense low-frequency engine vibrations that could be detected several blocks away.
  2. B
    Electric bus motors accelerate more smoothly than diesel engines, eliminating the sharp acoustic spikes associated with shifting mechanical gears.
  3. Because of their heavy onboard battery arrays, the electric buses weigh considerably more than the diesel buses they replaced, causing their tires to produce more road-contact noise than the motor noise eliminated.Answer
  4. D
    All municipal transit authorities that transition to electric bus fleets inevitably experience unforeseen secondary operational costs.
  5. E
    The acoustic monitoring equipment did not measure noise levels on nearby residential side streets that were never served by the express bus routes.

Answer

The statement explaining that the heavier weight of electric buses causes tire-contact noise that exceeds the engine noise eliminated most effectively resolves the discrepancy.
The correct response provides an unstated causal factor that directly accounts for the increase in total noise: vehicle weight. Because the heavy battery packs make the electric buses significantly heavier, the noise generated by their tires interacting with the pavement increased by an amount greater than the noise eliminated by replacing the diesel engines. This allows the stated facts to coexist: the motors are quieter, traffic parameters remained constant, yet total measured roadside noise increased.

Step-by-Step Solution

1
Identify the two competing facts in the stimulus.
Fact 1: Buses with substantially quieter motors replaced loud diesel buses while speed, volume, and pavement remained constant. Fact 2: Roadside ambient noise levels along those corridors increased slightly.
Resolving a paradox requires clearly isolating the two seemingly incompatible premises that must both remain true.
2
Formulate a bridging mechanism that allows both facts to coexist simultaneously.
Look for an auxiliary factor introduced by the electric buses that generates new noise from a source other than the motor (such as weight, aerodynamics, or tire friction) without altering speed or traffic volume.
An effective resolution must explain the increase in total sound output without denying that the motors themselves are quieter.
3
Evaluate the choices to find the one providing this bridging causal mechanism.
The explanation regarding increased vehicle weight from battery packs causing greater tire-surface noise accounts for the net increase in overall sound levels.
This bridges the gap by showing that motor noise was replaced by a larger amount of tire-friction noise.

Key Concept

Reconciling Apparent Discrepancies via Unaccounted Compensatory Mechanisms
Estimated Time:1m 15s
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Topics

Resolve the Paradox

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