First bicycle taillight and flag are interchangeable as long as there are some equipment or some way to attach my bike light at the end of the pole ; or perhaps even better there's a way to attach the light on top or below the flag, to have them both.
I have a bicycle trailer Burley Travoy and a taillight Blackburn Mars 4.0 which I both enjoy. I usually wear the bike light on my backpack as I'm also an inline skater and the light is higher that it would be attached on the seatpost, almost a foot higher! There's also trucks on the road with a higher cabin, and I'm much more afraid of trucks than cars. I enjoy my current setup.
However when I have the bicycle trailer on, my light is 2-3' further than usual ; and if a vehicle hit my trailer, it's pretty sure I'd fall of my bicycle and thus have a chance of falling toward the traffic instead of the safer side of sidewalk/parked vehicles/ditch. So having my light around the height of the trailer wheels instead of the bike back wheel would be safer ; or at least have a reflective flag. Attached are some ideas of how a flag could be installed on a dolly-like bike trailer, which in my case is the Burley Travoy.
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Also a light on top of the trailer wheels would be better to be wide angle or 360 degrees as when one turn with a trailer, an angle form at the point of attachment ; similar to when a train take a curb. Attached are angles showing both turning side with a 2-wheeled bike trailer.
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Also attached is a setup with a 360 degree light ; imagine a a 1' thick cutout from the shining part of a camping light, which would amount to a cylinder like a revolving restaurant flooded with light (from inside).
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So the best setup I can think is the active part of a camping light at the top of flag straight up from the trailer wheels, the orange flag in my sample.
Thanks for helping me be safer when riding on my dolly-like bike trailer.