ORD T1 checkpoint - the one to the far left between T2 and T1

AIT (I believe it is the backscatter with the two boxes in use)
People ahead of me in line - 10

The first 3 were sent through AIT, then what appeared to be a supervisor called out 'rotation!' and the next person was sent through WTMD. The Rapiscan then apparently had a failure, and all activity came to a standstill for several minutes. Finally the other scanner lane was opened, and passengers advised to move their luggage to the other scanner.

The next set of passengers were all sent through AIT, except for a woman with a tiny dog who actually wanted to use the AIT with the dog and had to be redirected to WTMD.

My turn was next and I was prepared to be sent through the AIT, and to refuse because I have a valid medical reason for doing so. Before I could say anything, the 'supervisor' directed me through the WTMD.

I do not know why I was sent through the WTMD rather than the AIT; the next several behind me were also sent through AIT. It may be that the 'supervisor' was actually perceptive and noticed that I would not be able to use the AIT, although I had removed any visible devices before security. It may be that they didn't have female staff to do an assist. It may just have been random.

My overall impression was that the screeners were in contrast to my usual European airports unprofessional. A few were obviously embarassed when the scanner came to a halt, and to their credit tried to lighten up the mood by joking, but the jokes were not necessarily appropriate in that setting (at least one is used here on TS&S) Signage was in place on the machine, below my eye level, but same as the sample which ORDofcr posted. I saw no other opt outs, and the supervisor did continue to explain the AIT in a rather vague fashion, but nobody asked questions etc.

Considering the very small number of people ahead of me, it took at least 2-3 times as long to clear security as it does elsewhere with that amount of people.