I have found two arguments that I am going to talk about and I hope these are close to the answers as possible that you want. The first one since it is the very nature of terrorism not only to cause immediate damage but also to strike fear in the hearts of the population under attack, one might say that the terrorists were extraordinarily successful, not just as a result of their own efforts but also in consequence of the American reaction.
The second argument: Since the victims of car accidents come from every geographical area and every social stratum, one can say that those deaths are even "closer to home" than the deaths that occurred in New York, Washington, and Pennsylvania. Most say that the September 11, 2001 attacks can only be compared to the attack on Pearl Harbor.
The first premise: by implementing practices and requirements that are directly related to the results, rather than throwing the nation into near panic and using the resulting fears to justify expensive but not necessarily effective or even relevant measures.
The second premise: if we have been able to treat the terrorist attacks of 9/11 in a way similar to how we treat the carnage of the nation's highways. The conclusion: we might have accomplished something. In my opinion the stated premises support the conclusion. The idea of implementing practices and requirements directly related to results, or using the tactic that will not require so many unnecessarily expenses is a good support of the conclusion that it could be an accomplishment. The true result could not be obtained by using this method and was very weak in its attempt. The premises add probability to the conclusion. You never know what would have happened in case of implying new ideas...