I ran across several rattlesnakes when we lived in Colorado. The first one, I was hiking down a single-track trail on a hillside, and the snake plopped down on the trail behind me and started rattling. I guess I had walked right past it. That gets your attention in a hurry. In that area, they are some sort of Prairie Rattler that doesn't get too big- maybe 42" or so, not like the eastern/western diamondbacks.

Around here, all I've seen on trails have been little snakes about a foot long of unknown species, and the green ones about 18" long. Some on dirt trails, some out on the MUP.