What I posted in the other thread:
http://www.bikeforums.net/showthread.php?p=8399500

Turn signals work best when there's somebody nearby -- normally to your rear flank -- and is there long enough for you to look, signal, then start to make your move.

But, cars aren't nearby for more than a couple seconds. They're either passing you or you're passing them.

And, if you're moving across a lane or three, and you're going significantly slower than approaching traffic, you're running a real danger of getting smushed no matter how well you're illuminated.

The safest time to change lanes, then, is when there's nobody close enough to matter. It would be virtually impossible for you to get hit, even by a totally clueless driver.

Since there's nobody close enough that they need to see your turn signal, that means that turn signals won't be of any use.

IMO -- save your money, and ride smarter.