How nervous does other people's driving make you?

That is, while you are the passenger with them.

This is prompted by a situation I was in last night…my boyfriend and I went to Telluride this weekend which is in the southwest corner of Colorado. So to get back to Boulder we obviously had to go through the mountains. The drive there was fine and the weather was good, but last night we got near Vail and the roads became horrible, it was dark out, his headlights were barely working unless they were on highbeam (and when your highbeams are on and its snowing you can’t see either), and we were both very tired. Normally, snowy situations don’t frazzle me, but last night I just couldnt take it. I refused to drive. I couldn’t keep my eyes open either. So he woke up and drove and the entire time I felt like I might die any second. Then we hit heavy fog and I was positive my life was going to end. It was a really bad experience. I’ve never been so scared in my life, and I am otherwise just about the farthest thing from a worry wart in the world.

So I just wonder, are you paranoid when other people are driving? To what degree? Does it depend on the situation? How long you’ve known the person? Or does it totally vary depending on the person? I’m curious.