The way the system is set up, all airports are only as secure as the least secure airport. Once you go through security, you can fly around the country from one airport to another without going through again. Thus, if the tools were effective but you somehow made an exception for small airports, you just invite terrorists to use those small airports for sneaking their weapons through, which can then be used on bigger, more visible flights later.

I think these machines are a waste of money and the threat of terrorism vastly overblown in general, but if you make the assumption that the machines are desirable and useful, you can't make an exception for small airports.

But the point is alternate and arguably more effective methods exist, particularly scaled to a smaller airport. I would say that even if you patted everyone down at the airport, the cost would be less but that's just speculation on my part.

However, I would like to post an excerpt from the last link that I shared.

"TSA acknowledges that it takes 10-12 seconds per passenger to go through the scanner while only 2 seconds is needed for the metal detector. Since the scanners have a 54% false positive rate, approximately half of those using the scanners receive some form of pat-down, further slowing the process. When hundreds of people are in line, that additional 8 to 30 seconds each adds up, resulting in substantial delays.

So while TSA claims that the scanners speed up the screening process, the opposite, in fact, is true."

54% is an astonishingly high number. Any idea what the false negative rate is? I wonder if the scanners are any better than just flipping a coin....

Well, a false positive rate of 54% essentially means it is in fact just coin flipping.

For any other product on the market, a failure rate of 54% would be cause for a complete recall if not a wholesale scrapping of the product but apparently does not apply in this case.

You need to know the false negative rate to evaluate it against a coin flip. For example, a hypothetical machine with a 50% false positive rate but 0% false negative rate could still be really useful, if the machine itself is cheap/fast and the subsequent screening slower/more expensive. With such a machine, you know that the 50% who pass are clean, thus reducing your followup screening load by half.

Of course, a machine with ~50% false positive and negative rates is indistinguishable from flipping a coin.