PrepTest 129

Incorrect. The argument relies on the speed limit having an effect, so it's assuming that cars before the implementation of the 55mph speed limit were going faster than that. As such, cars being capable of going faster than that is something of a necessary assumption—certainly not something that weakens the argument.

Incorrect. Both the premises and conclusion of the argument are about the highway accident rate, and there was no speed limit there, so these roads can't be highways and thus are out of scope.

Incorrect. The important question is whether people are driving slower than they were before there was a speed limit. If that 55mph speed limit took people from 75mph down to 65mph, then even though people are still speeding it had an effect. This answer doesn't provide enough information to necessarily affect the argument so we can rule it out.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Weaken

Stimulus Summary:
No highway speed limit coincided with increases in accident rates. Implementing a highway speed limit coincided with lower accident rates. Therefore, lowering the speed limit caused the lower accident rate.

Answer Anticipation:
This is a Weaken question, so you should always be on the lookout for a correlation/causation flaw—the most common flaw in this question type. Even if you didn't see the correlations in the premises, that conclusion should have made you think causality—something "has reduced" the accident rate is a causal statement.

The causal relationship from the conclusion is that lowering the highway speed limit caused the accident rate to go down. Weakening arguments that jump from correlation to causation is generally done in one of three ways:

(1) Identify an alternative cause (maybe more cops were placed on the highways and thus people drove safer because of that)

(2) Counterexamples (places where the speed limit was lowered and accidents didn't decrease; places where accidents decreased without lowering the speed limit)

(3) Reversed causality, which doesn't make sense here because the speed limit was lowered before the accident rate went down

With that in mind, time to head into the answer choices!

Answer Explanation:
This answer raises an alternative cause for the lowered highway accident rate. If there are fewer accidents because people can better control their cars when in danger, then the speed limit isn't the true cause and the argument is weakened.

Key Takeaway:
When dealing with a Weaken question (or a Strengthen question), the most common flaw is correlation/causation. You should be on the lookout for this flaw in this question type, and you should be happy when you find it—you know the types of answers to look for!

Incorrect. This answer can feel as if it's introducing an alternative cause, similar to (D). However, note the features here—seat belts and airbags. These don't prevent accidents, they prevent injuries for someone in an accident. As such, these features aren't an alternative cause for a lowered accident rate, and this answer is out of scope.