Generally, I prefer to ride alone. That's because most of the boarders at my current barn are scared to canter and scared of trail rides, so they just want to go for a walk on the trails and chatter the whole time.
Right now, the trails I ride on aren't much. If something bad were to happen and I were to disappear, for the majority of the time I am within sight of the barn. If my fool of a horse bailed on me, he'd go home. I keep my cell phone on me (strapped to my boots) so if I find that I can't walk back, I can call the barn and someone can come out on the RTV to get me in a few minutes.
Now, there's a few things that I have been considering for when I switch barns. I have an iphone. I have the "find my phone" app, and my husband has the password to it. If I vanished, he could use that to track the location of my phone, which would hopefully remain with me and working! Tell people at least the general direction of where you're headed. Mark your horse with some kind of tag that has your contact information.