Gotta agree, the flag will be useless when you are moving, and especially at night. You need something reflective and more likely to hang down to catch headlights.

Since you've gone to this extreme... I have one more suggestion... something I saw one time that really made an impression on me... a light shining on your back. I ride this arterial road with a speed limit of 50MPH, and one night I saw a cyclist ahead of me that instantly said "cyclist." He had a light on his back rack, aimed at his back... there was no question about what he was, there was no mistaking him for a motorcycle or any other object on the road.

At the time I was using a triple belt beacon (incandescent bright random flashing) setup with a red led flashy, so I felt pretty good... but when I saw his setup, I realized he had trumped me.