Checkpoints and Borders Policy Debate - Why do some people get metal detectors, others millimeter wave/backscatter?

Recently at DCA and PHX, I've noticed that TSA will have both a traditional metal detector along with a millimeter wave or backscatter machine open in the security check line, but often the TSO will tell all passengers to go through the new machine, leaving the metal detector open but not in use. Then, every so often (has happened to me before), a person will be pulled from the new machine line and told to walk through the metal detector. Often the person allowed through the metal detector is an airport employee, but often it is just a regular passenger like me. Is there any rhyme or reason as to who gets to go through the metal detector and who must go through the new machines? While I typically avoid having a conversation with the TSO, is one allowed to ask the TSO to walk through the metal detector rather than the new machine?

While I typically avoid having a conversation with the TSO, is one allowed to ask the TSO to walk through the metal detector rather than the new machine?

You are allowed to ask, but doing so pretty much guarantees that you will be sent to the NoS instead of the metal detector (presumably because you are 'testing' the system because you have something that the NoS will catch and the WTMD will miss).

Of course, this logic doesn't seem to work the other way - I believe all requests for NoS instead of WTMD will be gladly accommodated, even though there are things that the WTMD will catch and the NoS will miss.

Welcome to FT. One of the first things you'll learn reading this forum is that there is no rhyme or reason to the actions of TSOs. Chollie is absolutely correct--a request to go through the AIT instead of the WTMD will be granted, whereas a request to go through the WTMD instead of AIT will likely guarantee selection for AIT, and might even be treated as an opt-out regardless of whether you'd been given instructions as to which direction you should go in.

When I've seen someone selected for the metal detector (including myself) it usually looks likes TSA did it just to move people through security quicker.

The last time I was selected, a person who kept finding things that needed to be put through the X-Ray machine was blocking the backscatter machine and causing a long line to form (and bags to back up on X-Ray machine). The TSA started directing people through the metal detector until the backup subsided.

Never asked if that was their motivation, and not saying there aren't others, but that's what I've usually seen.

I believe they also send families with children under a certain age (12, I think?) through the WTMD instead of the WBI machines. I've seen all children and two parents of opposite sexes sent through the WTMD in those cases.

When I've seen someone selected for the metal detector (including myself) it usually looks likes TSA did it just to move people through security quicker.

The last time I was selected, a person who kept finding things that needed to be put through the X-Ray machine was blocking the backscatter machine and causing a long line to form (and bags to back up on X-Ray machine). The TSA started directing people through the metal detector until the backup subsided.

Never asked if that was their motivation, and not saying there aren't others, but that's what I've usually seen.

I have noticed the same thing at ATL but generally people are sent to an adjacent line with the metal detector when things get clogged up. Less frequently I have seen them direct people to the metal detector right next to the backscatter when dealing with people who slow the line down.

Welcome to FT. One of the first things you'll learn reading this forum is that there is no rhyme or reason to the actions of TSOs.

This. Particularly the latter.