It normally didn't last long but I remember some of the fog up in Northern Wisconsin was like driving through pea soup. It was always nice when it started freezing. Like someone else said, I80, I81, I84, and especially the Pa Pike get pretty bad too. Town Hill just south of Breezwood seems to be foggy even on a clear night as do the rest of the hills on I70 till you get to Frederick, Md.
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Where have you had the most trouble with fog ? 
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