If you carry anything, do not use cable ties to secure it. I did this once taking a golf club into work for a friend. Although I did the 12 mile commute 3 times a week, on this occasion I was absolutely knackered when I got work, so much so that my colleagues were very concerned for me. Turned out that when I applied my brakes at the junction at the bottom of my road, the cable tie pinched the brake cable, this meaning I did the whole commute with my back brake on! Only found out because I had to lift my bike up some steps at work, and when the back wheel bumped on them it hardly spun. Lesson learnt the very hard way!