Ah, "safe" backpacks.
No such thing. Dress for the slide, not the ride. You're only thinking of one aspect of safety: Being seen.
Before you ask, yes, I do wear a pack. But, I also wear a hard shelled spine protector under it. Why? Should I go down, the hard shell spine protector will "hopefully" keep my back from bending it ways it shouldn't with an extra ball of "stuff" strapped on my back.
Some of you are students and you will have hard things in your pack like binders, books, maybe even a laptop. Even shoes or wads of cloths will cause your back to bend in odd ways..... Consider extra spinal protection if you're going to wear a pack....the foam "back protectors" that come in some jackets don't cut the mustard. I have personally seen someone go off the road, over the edge and hit BACK FIRST into a tree, get up and walk away because he was wearing a back protector. Books/laptops/whatever else hard you have is the equivalent of a tree if you go down and land on your back, even clothes only compress so far.
Something like this:
Safety speech over!
You could get something like this that has retroreflective parts sewn in for maximum visibility. On sale, $50 right now.
http://store.nike.com/us/en_us/pd/ma...c-a29db3983bf5