Bruce,

There are limits to what can be done. We are limited by techonology and human nature. We are limited to measures that can be applied to close to two million passengers per day without causing untollerable delays. On the other hand, we have to take whatever reasonable precautions we can to prevent additional attacks.

Considering the many contridicting goals they are trying to acheive, and that they are a government agency, I think they're doing a pretty good, though not perfect, job.

Having a random element is completely appropriate in that it adds unpredictability and is a recognized means of backup quality assurance.