Hmmm. I have no doubt they make you more visible but I have a contrarian theory about that.
All those people who brush too close to you. Do you think it's because they didn't see you or do you suppose it's because they are trying to intimidate you? If you, like me, think it's the latter consider this: How do you suppose they choose their targets? Do you think those yahoos would be more inclined to assault a confident-looking rider in everyday clothes or the one with safety flags, winky-blinkies and wearing the dayglo shirt? Which one do you suppose they'd think would be easier to intimidate?