More wild guessing - mightn't it also be that there are large objects whizzing down the interstate, creating wind patterns that may blow the fog clear of the lanes? If the fog is as dense as you describe, it's probably not moving much, so the wind from speeding cars could blow it off the road and it would just collect wherever there isn't such wind.

I recognize that this means some people have to go fast before the fog clears, but as we've heard from others here, that happens.

Here in San Francisco, we get plenty of fog, but it tends to be higher up and just cools the area down in the afternoon, rarely creating such a driving hazard. It is beautiful when it comes through the Golden Gate, though, as the Gate forms it into a long, thick bar of fog that extends across the bay, while the sun shines all around it.

The hazard here is the rain - we only get rain in the winter, and each year, when the rains first start, people take no heed of it as they're driving and we seem to have a bunch more accidents on the road. I describe those early rains as "stupidity falling from the sky."