I think it could be argued that you weren't actually "following" AT ALL. Lets say the truck was stopped or driving slowly. Since you were in a different lane all together (initially) the definition of following would not apply to you. If you moved over and struck the truck due to a "sudden" appearance of heavy fog, then you still weren't really "following" him, in my opinion. To me, "following" would have to mean "for some notable distance".
I know this charge is laid all the time, in this and similar circumstances, and it is quickly dispatched in court by knowledgeable lawyers and paralegals.
On a personal note, fog is my greatest fear when driving out on the open road. The most horrific motor vehicle accidents ever recorded were caused by sudden, unexpected fog banks rollin' in to wreak havoc. I am so "white-knuckled" when driving through fog, it takes a few moments to release my death-grip when I reach my destination.
I, personally, don't agree that a charge NEEDS to be laid at EVERY accident scene. Our Canadian weather throws a LOT of curves at us poor mere mortals, and just maybe we need to be cut a little slack in situations like this.
Here is a really good article published after that big 87-car pileup on the 401 several years ago. Note how quick the coroners jury was to blame the problem solely on speed and suggested photo-radar will solve the problem, while an actual traffic researcher scoffed at the notion that speed alone is the problem.
http://www.drivers.com/article/324/ Clearly, the best solution is for all cars (not just high-end stuff) to come equipped with infra-rad detectors which can scan for problems in the fog that the human eye can't see.
When driving in fog, I am constantly worried that I am going to be rear-ended. Trying to find a balanced speed in the fog, to protect yourself from both the front and the rear, is, to me, the most difficult aspect of driving... PERIOD! Even stopping on the side of the road is a crappy option, and many people have been hit hard by doing so (even snowplows kill people stopped on the side of the road).
To trivialize the difficulty of handling thick fog by slapping some poor sap with a ticket (on top of his totaled car and probable increased insurance rates) is almost cartoon-like when you think about it.
Just get the proven, working, current technology out there in our cars and maybe we can actually stop these "ACCIDENTS" from happening.