I did not assign responsibility to the rock.
Again, I point to the RCW (revised code of washington) which calls impeding traffic in anything other than the right lane an infraction. So someone sitting in the left lane doing 10 under the limit when the flow of traffic is 10 OVER the limit (making the differential 20MPH) is in violation of the law being there; in the case of the van I saw with no lights on, that's two violations (driving without illumination is another violation.)
There are turns in the road that hide some things in the distance and aren't marked with a yellow caution sign either.
You're right, doing 10mph under the speed in the left lane of a two lane highway is kind of lame thing to be doing. Hell, like you say, it might even be illegal and a cop might pull them over and give them a ticket (I dont think it is slow enough personally, but let's roll with it).
The point here, is that when you crash into this guy doing 10mph under the speed limit, he's doing something slightly wrong, you are doing something HORRIBLY wrong. I don't know under what conditions you could crash into someone doing 10mph less then the limit that is otherwise driving steadily and safely, and have more than 50% of the blame fall onto him.
Now the bad lights changes things a bit. If both his rear lights are out then you may have a case. I would still argue that you shouldn't have crashed into someone only doing 10mph less then the limit, unless there's some other information im missing, but you might actually have a case.