GamblerGORD53

I think I understand what happened. I heard that it started at the start of the bridge, which makes sense.
The pack was just getting started, so was bunched and 3 wide. They would be talking to each other. Since few if any have a mirror, then only the back guys would notice traffic coming. The shoulder was good enough for one line of bikes.
Then for 100 yards the shoulder narrows for the barrier, before the bridge. So the bikes have to squeeze closer to the lane line.

The truck is pulling a trailer, somebody said. Likely would be a wide work trailer. The inner left pavement has but a few inches outside the yellow line.
So a suicide squeeze happens, where neither can give way and both are using the full lane. So the blunder just happened.
IF there was a trailer, then the delayed reaction was also inevitable. That could explain the length of the mayhem. Only the ones by the lane divider line were hit.

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Then I looked at the RCMoeur link to the historical aerial photos. EXACTLY where I start the accident, is a SUV pulling a TRAILER. It's right wheel is within a foot of the right side lane line of the left lane.