It all comes down to cost benefit analysis. Safety, as defined as the absence risk, is unattainable. One can, however, take reasonable precautions to avoid known risks. Take, for example, the 9/11/2001 terror attacks - the basis for much of the Patriot Act; they involved the coordnated planning of at least 20 people, armed with boxcutters, in a plot to drive commercial aircraft into buildings. Much emphasis was then placed on how to prevent such an attack in the future. My personal preference was simply to work harder on the airline security aspect, secure the cockpit cabins and arm the crew, but many chose to try to use greatly expanded SIGINT to try to discover the identity of all terrorist agents (in the U.S.) before they could execute a similar terrorist attack, even if that involved using a completely different method, say the coordinated poisoning of turkeys just ahead of Thanksgiving.
