+1. I've also noticed that two blinkies, one low and one high are most noticeable in urban traffic. I once followed a bicyclist who had a single blinkie on the back of his helmet. It was helpful, but I didn't notice it right away because my attention was lower, at car tail-light level. So I'd say having one at seatpost level is the most helpful position, followed by helmet, then all the rest of the possibilities for true UFO styling.

My three blinkies (two on rack, one on seatpost) are too close together. Time to get one on my helmet.