I often ride alone and in places that are not well populated -industrial areas and relatively isolated paths. I suppose I've never felt unsafe, though if there were reported attacks - like there have been in Australia then I would avoid the area when alone. Yes- on a bike you are kind of vulnerable, if someone knocked me over I probably would not be able to get untangled from the bike and up all that easily and would be easily overpowered. If I thought about it too much I might not ever go out....but I have to live my life so I guess I try to not think about it too much.