Puget Sound has a lot of shipping traffic, and an active and professionally run VTS system. And we have radar on Eolian. So I was not particularly worried... focused and intense, yes, but not really worried.
As we started across, I notified VTS giving our speed and that we were going to be crossing the lanes. They, in turn notified us of shipping in the area. It looked good for us - a northbound freighter would be past us before we got to the northbound side of the Sound, and a southbound freighter would pass behind us as we entered the northbound lanes.
About the time we entered the separation zone (the "median" between the traffic lanes, if you will), a problem developed. We heard Manolo (the southbound freighter) complaining of a whole bunch of fishing boats out in the traffic lanes a couple of miles to the north of us, and then he said that he might have to veer into the northbound lanes to clear them.
I watched as Manolo's vector on the AIS display swung until it pointed right at us! Crap. The AIS app showed we were clearly now on a collision course, and given our relative speeds, it was going to be difficult for us to get out of his way on any course. So I contacted Manolo directly on VHF-13, and told him of our predicament, and advised that we were making a 180 degree turn and heading back west toward the southbound lanes (on what would be a collision course if had stayed in the southbound lanes) to try to get out of his path; he acknowledged.
Now came a period of tense waiting, completely blind in the fog.
Eventually, Manolo contacted us and advised that he was past the offending fishing boats and was returning to the southbound lane; I acknowledged and returned to our eastbound course.
In the fog, we never saw Manolo.
Oh sure, he was visible on the radar display, but that display does not show course or speed, only relative position. It was the AIS that made obvious what was happening. If you already have an AIS receiver onboard, you already know this. But if you don't, then I strongly recommend you get the Ship Finder app (there's a free version - that's what I have).