[KGIII]

Turn signals...

Could it just be a small and lightweight LED panel that flashed a bright arrow pointing in the intended turn direction?

With LED, it can be pretty efficient and still be really bright. I would think a blinking bright arrow would be fairly universally understood.

[stevenwoo]

One issue is that now a lot of riders are using blinking lights all the time when riding for visibility reasons (during the day), so the arrow would need to be big enough to differentiate and visible front and back so now we could be talking about something much more obtrusive. If they're on the back, there's a limit to how big one can go (on easy to mount positions) because the rider's legs/feet/butt will strike protruding things from the bicycle.

[KGIII]

Could they turn that blinking light into a an arrow at the appropriate time? I ride a motorcycle and have upgraded all my signal lights to the maximum legal output.

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.

[stevenwoo]

Those sound like good ideas, though something that big would still have an issue fitting on something other than a recumbent or cargo bikes (often see lights that size already on those now).

[KGIII]

I was thinking about that and it could just attach to the frame and then use something like an aluminum pole to hold it out over the rear tire. It'd be light and adjustable.

It'd look dorky, but it might save lives. I doubt it would add much weight or reduce aerodynamics very much, at least not as regular rider speeds. It's not like they would use it while racing.