Logical reasoning PrepTest 113 · Section 4 · Question 17

An increase in highway traffic would lead to an expectation that fatalities would go up, since more people are on the road. This answer rules out an alternative cause of the opposite effect, which isn’t what we’re looking for.

The argument doesn’t rely on a majority of drivers obeying the new limit, just enough to be responsible for the decline in highway fatalities. That may require only 10% of drivers to follow the new law!

To be a bit pedantic, the argument is about speed limits and fatalities, not driving speed and accidents. Ignoring those shifts (which we can’t do - they’re enough to invalidate this answer), the argument already established that there was a relationship between the two last year, as they both went down. It may not be a causal relationship, but a correlation is a relationship.

While the argument may assume that some people followed the new limit, that doesn’t need to be the result of increased enforcement. Additionally, if anything, the argument is assuming that the only relevant change was the speed limit decrease - an increase in enforcement of speed limits could itself explain the decline in fatalities, which would weaken the argument, so it can’t be something the argument takes for granted.

Taking a look back at the stimulus, the argument notes that the decline after the speed limit was decreased was compared to the “previous year.” If that year had an abnormally high number of accidents, the lower rate this year could have just brought it back down to the average level - and thus there may not even be a decline in fatalities to find a cause for! This answer rules out a possibility that would call the correlation into question, and it thus is something that the argument assumes.