I had a flag on my trike because it was low enough that I'd disappear between cars if I didn't. I'd be looking at doors, not windows. This was only a problem in traffic. In the country, it wasn't a problem.. but it didn't help much there either due to poor rearward visibility. But I was unique enough that it helped.

I haven't come up with a good way to put on on my SWB.. and haven't spent much time because I'm not sure it's necessary. I have a hi-viz helmet and a yellow seat bag on it.. so I'm pretty visible since my head height is about the same as a side window on a mini-van.