Big, bright arrow-shaped wireless turn indicator (rear only)
 

Emits beeping sound, and small light on remote lets you know it's activated. Manual turn-off, no front lights, or brake lights, but looks good enough to alert people behind you. Not sure on the effectiveness during the daytime though. I'm also curious on whether it is waterproof.

My bike has a rack in the way, so seatpost mounting is out of the question for me, though I could probably figure out something.

http://www.sharperimage.com/si/view/...-Signal/201372

Once again, I think it as a solution searching for a problem. A nice bright light may help cars to notice a bicycle, but turn signals seems to serve no purpose.

For the person who often encounters road surface conditions that make it dangerous to take one hand off the handlebars to signal a turn with a hand signal it might be a good idea provided the controller was mounted such that it could be easily manipulated with just a thumb while maintaining a firm grip with both hands.

Other then that, I think hand signals (especially left arm sticking out for left turn) are more visible especially if you wear a brightly colored/relective glove and/or arm band(s) or full arm length sleeve bright colored reflective shirt. I can certainly see situations though were they might be a good idea when riding through chuck holes or such on bad roads where you want to keep both hands on the bars and this would be better then not signaling at all.

What happens if two cyclists who both have one of these on their bikes are on the same road? Does one of the riders activating his wireless controller activate both of their turn signals?

It's the same story with cars really, light-up signals just let's people know that you wish to move over, but it certainly doesn't mean that anyone will let you.

The lights are way too close together. I doubt one could distinguish an arrow shape at 100 feet, let alone which way the arrow is pointing.

I think the best solution for someone looking for turn signals is to get some bright orange lights, and mount them to the ends of the handlebars pointed backwards.

Hand signals would still be needed with this light, there's no front blinkers.

They're more distinguishable than those goofy little red and orange things like from Bicygnals and Magpie.

Even better would be two sets of ambers on each end (which is how it should be). You would just need a power source and a way to turn them on/off.

Amish buggies are lit up like Christmas trees, just wait until something like this is a requirement on all road bicycles too. Not a bad thing overall but many people in cars don't use theirs so whats the point.

I do think they would be better understood than hand signals.