What do you think of when someone mentions a 'dangerous road?' Some twisty mountain highway? An ice winter track? A high-speed interchange? Maybe you should think of a normal highway, just with a lot of fog.
The real danger of a foggy road is the huge loss of visibility, coupled with how cavalier drivers can be though they can't actually see anything up ahead. Drivers often fail to go cautiously enough, and one small accident can easily turn into a huge pileup, as you can see in this video, or this one from four months ago of a grandma not slowing down before hitting a pileup to see what I mean.
Some of the biggest car crashes in recent history came down to fog on the German Autobahn (ones that make wrecks like this seem small), so be careful when heavy weather rolls in on your local highway.