[RegulatorCountry]

I've always associated large pileups with Florida, smoke from forest fires used to do it more often than not. Particularly fog prone areas in NC have digital speed limit signs with visibility sensors, on mountain interstates at least. I've seen them reading 5 mph before, it's vertigo inducing, fog that thick.

[Lancey Howard]

Particularly fog prone areas in NC have digital speed limit signs with visibility sensors

LOL!
Do people really need speed limit signs to have half a brain when road conditions are crappy?

[Verbosus]

Twice I have seen a fog hit so quick and heavy, that it obstructs your visibility
to half way down your hood, and within seconds.

Once it was in a very rural area northwest of Houston by about an hour - but
in a low lying area, and the other time it occurred about 45 miles outside of
Amarillo, Tx.

Both times it lifted enough to see 10-15ft past the hood within minutes,
but both hit so rapidly there was barely enough time to pull off onto the shoulder.