I've been to all three checkpoints at Dulles: east & west main checkpoints and the Diamond Lane very recently. The general set-up is a magnetometer right next to a Cancer Machine. The Diamond Lane has one of each. Here's my advice and an observation:

1. Avoid the Diamond Lane. You have only a 50/50 chance of getting selected for the Cancer Machine. If you can, I would stop across the hallway from the Diamond Lane and pretend you're on the phone inorder to be able to observe if the Cancer Machine is in operation. If it is, or if you're not sure, head for one of the two main checkpoints.

2. Main checkpoints: The west checkpoint has the Cancer Machines to the right of the mags. The opposite set-up exists at the east checkpoint. After clearing the ID checker, simply go into a line heading towards the magnetometer. Each scanner and mag line has its own carry-on x-ray. There is a chance that the moat dragon will divert you to the Cancer Machine, but, there are usually enough sheep lining up that it's not a problem. Your chances of getting through without being irradiated and/or groped are significantly less at the main checkpoints.

Observation: As another FTer has posted, I also noted that the lines for the magnetometers were 2-3 times longer than the lines for the Cancer Machines. There were no opt outs at the machines because people had already opted out by choosing the lines that they picked. Unfortunately, the people who got in line for the Cancer Machines appeared to be our foreign guests, who were probably unaware of what was about to happen to them.

If one conservatively assumes that the mag lines were only twice as long as the Cancer Machine lines, 2 out of 3 people opted out. When given the choice, humans don't normally pick the longest line. So, self-opting out can be the only reason for voluntarily getting in the longer of two lines.

You will NEVER see this observation in PV.

The next time I'm at IAD, I plan on making a photographic record of the line difference - WTMD and NoS - so that I have irrefutible evidence in response to the usual TSA lies that few people opt out of being subjected to a virtual strip search.

FWIW, I noticed this at ORD with a number of Asian passengers..

Hit the trifecta at Dulles today. At 3:10pm, encountered an almost vacant Diamond lane, the NoS not being used for anyone ahead or behind me, and a fairly attractive and very pleasant WTMD operator. Unreal.....

For the West side main lines, self-selecting the WTMD side or Pervo-vision side doen't work any longer. Yesterday morning they were funneling 80% of the passengers for the strip-search, and very few for the WTMD regardless of which X-ray you chose.

I will add that on a normal morning they would have 6 X-rays open, there were only four - causing a long backup in the queues. There were also more screeners standing around. I have a theory that they de-staffed those two screening lines in order to run the Rapescans.

Maybe they don't do it all the time, but when the lines are longer, TSA funnels people to specific lanes and appears to do it so that the total elapsed time from the start of the lane to the end of the process is roughly the same. My goal is fastest time out to the gate, so Double Diamond almost always the best, but it's not as though one can compare at any given moment without spending 20 mins. walking the airport.

I saw that at SAN T1 last week. For a good while, they only had 1 ID checker and 1 NoS (BXR). Was that way for quite awhile. I talked to a pax about my shirt. Anyway, as I got closer to the actual CP, they finally opened a WTMD line (BXR roped off). I, of course went in the WTMD line and the guy I was talking to went to the NoS line. Interestingly enough, TSA had 2 carryon x-rays scanning bags, but the bottleneck was the NoS. I whipped right thru the WTMD line. Didn't see the guy on side of the line before I left. As far as I could tell, he was still waiting for the NoS.