Logical Reasoning

PT129 · S2 · Q20 In the past, when there was no

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In the past, when there was no highway speed limit, the highway accident rate increased yearly, peaking a decade ago.

Conclusion

Setting speed limit at 55 mph has reduced accident rate by at least 15%

Evidence

Every year since the 55 mph speed limit was introduced has had an accident rate at least 15% lower than the peak rate of a decade ago (when there no speed limit)

Evaluation

This is a Correlation to Causality type argument: the introduction of the 55 mph speed limit coincided with ten years in which the accident rate was at least 15% lower than the peak year right before we had the speed limit. The author then becomes firmly sure that speed limit is the causal reason for that. We are supposed to consider the plausibility of the author's story line (does anyone care about the speed limit? do they see the signs? does it affect their behavior), but more importantly we're supposed to consider whether there is some other way to explain the curious fact. How else could we explain why the ten years since the speed limit have had a 15% lower accident rate?

Maybe there are fewer drivers .. safer cars ... we can't argue that this is just a "regression towards the mean", meaning that ten years ago when we had that peak year it was just anomalously high, because they told us that the accident rate was steadily climbing each year prior to that speed limit.

Goal

We would primarily look for some other way to interpret that 15% lower (so that it doesn't seem like the speed limit has caused the reduction), but we can also like any answer that makes it seem less plausible that the speed limit is having an effect. There will probably be some other change that occurred around the same time as the speed limit was instituted.

20.

Which one of the following, if true, most seriously weakens the argument?

  1. In the years prior to

    Opposite

    This somewhat helps the author. In order to believe that a 55 mph speed limit is making the roads safer, you have to assume that prior to the speed limit some people were dangerously exceeding 55 mph. If there were no cars that could even go faster than 55 mph, then why would we think that speed limit would change anything? Since this goes the opposite of an objection, it strengthens somewhat?

    0% picked this

  2. Ten years ago, at least

    Out of Scope

    Out of Scope: "areas with a speed limit of under 50 mph" This argument is only concerned with the accident rate of highways, which previously had no speed limit and now has a 55 mph speed limit. Information about other areas of town isn't germane enough to what we're analyzing: why was there a decrease in the highway accident rate?

    12% picked this

  3. Although the speed limit on

    Too Weak

    This kind of undermines the author's story that a 55 mph speed limit has slowed people down, lowering the accident rate. The problem is that even if 51% of drivers go over the speed limit 51% of the time, there's still a lot of leftover drivers who may have slowed down and are now obeying the speed limit and causing fewer accidents. The author only needs to explain a 15% reduction, so even if a non-majority proportion of drivers obeys the speed limit now, that could work. Moreover, it could be that a 55 mph speed limit is frequently exceeded, but that drivers have still slowed down. Maybe when there was no speed limit, cars would average around 75 mph, and now that there's a 55 mph speed limit, they've slowed down to an average of 65 mph. That would still enable the author to say that the speed limit made the highways safer.

    9% picked this

  4. Correct

    Thanks to changes in automobile

    Why this is right

    This says, "Something else also changed ten years ago .... cars got easier to control in dangerous situations." This advantage in driving through a dangerous situation could easily be the reason why the accident rate is 15% lower now.

    Skill tested: Weaken · how this choice captures the argument's function is the move to repeat next time.

    67% picked this

  5. It was not until shortly

    Out of Scope: "injuries"

    This is also a change that is contemporaneous with the speed limit, but it would only have an effect on injuries / fatalities. It doesn't have the means to explain why there are 15% fewer accidents.

    11% picked this

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