Schneier on Security

Chapter 3. Airline Travel

Airport Passenger Screening

Originally published in Wired, 23, March 2006

It seems like every time someone tests airport security, airport security fails. In tests between November 2001 and February 2002, screeners missed 70% of knives, 30% of guns, and 60% of (fake) bombs. And recently, testers were able to smuggle bomb-making parts through airport security in 21 of 21 attempts. It makes you wonder why we're all putting our laptops in a separate bin and taking off our shoes. (Although we should all be glad that Richard Reid wasn't the "underwear bomber.")

The failure to detect bomb-making parts is easier to understand. Break something into small enough parts, and it's going to slip past the screeners pretty easily. The explosive material won't show up on the metal detector, and the associated electronics can look benign when disassembled. This isn't even a new problem. It's widely believed that the Chechen women who blew up the two Russian planes in August 2004 probably smuggled their bombs aboard the planes in pieces.

But guns and knives? That surprises most people.

Airport screeners have a difficult job, primarily because the human brain isn't naturally adapted to the task. We're wired for visual pattern matching, and are great at picking out something we know to look for—for example, a lion in a sea of tall grass.

But we're much less adept at detecting random exceptions in uniform data. Faced with an endless stream of identical objects, the brain quickly ...