This is a concern of mine as I travel through a dark area that is part of a bike path but behind a supermarket and a crappy area. I've already seen creepoids on the path in broad daylight.

I have a personal alarm I carry with me given out for free at a Women's safety thing. It isn't meant to call for help, it's meant to scare the hell out of an attacker (it has a really high pitched whine and flashing lights) - giving enough time to get away. I feel better carrying this. I also ride a lot faster through that area. And, I have lights on my bike and helmet.

Someone told me, if you are ever attacked, HOLD ON TO YOUR BIKE and make a lot of noise.