This is a tough one, because it's hard to prove a negative. Here, we've examined the instances of terrorism that were not prevented, where the system failed. I have heard of a few instances where terrorism was prevented, but it is usually a prelude to a more stringent security measure. "We almost didn't catch the underwear bomber, so we need more underwear scrutiny."
It may be that the measures are extremely effective and that terrorism is actively prevented every day, but we just don't hear about the successes. It might also be the case of having a banana in your ear to keep away alligators. "Why do you have that banana in your ear?" "To keep away the alligators."
