I would probably test to see if changing the colors was also effective. I don't think it would have to all that large, maybe half the size of an automobile license plate. When it is just going down the road, flash white. When it is turning, flash orange or red - in the shape of an arrow.
I'd test that and see how people responded to it. It might not even have to be that large. I'd just want to make it large enough that the arrow can be made out clearly and from, say, 100 feet away. The changing color may also help.
I suppose orange would be fairly standard and might be a good color choice to try.