As I was driving my car home today (haven't commuted much this winter ) after work it was dark and snowing. I saw a commuter with two bright blinkees. One on the rear rack and one in the middle of his back (I think on a back pack but he was a bit far away to see). The second light made him much easier to see. I think part of the reason was that one of them was pretty high up. I personally use two lights. One on my rear rack and one on my helmet. I like having one higher up and others have mentioned, if one goes, I have a spare.