I wore a backpack everyday (year round commuter) through my 6 years in college and carried light to heavy loads. I should also mention that my longest stretch I'd ever be on a bike was about 4, maybe 5 miles - mostly 2-3 miles. It never bothered me one bit, nor did I ever think it increased my risk of falling - I would think that if you like bikes enough to post on this forum, you're not at any more significant risk of falling while wearing a backpack compared to panniers. Now that my commute is about 8 miles one way, I'm using a rack trunk and so far, I can get everything in. I also hope to get a set of panniers soon. I do like feeling more free, by not having anything on my body.