Well, I run into 18 inch Rattelsnakes from time to time when mountain biking or nature walks. I don't freak out as they're usually very timid.
They rattle their tail when passing but since they're small and their strike distance is very short, it really isn't an issue unless you run directly onto them.
I stay in the middle of trails, b/c it has been my experience they like the bush for shelter.
........now Cobras are a different thing. I saw them in India while on vacation and they can move pretty fast and are more aggressive. Plus their strike distance is respectable.