FROM THE INTERNET...

Coming from England to the U.S. I was taught somewhat different hand signals. I have continued to use my U.K. signalsduring the 18 years I've been cycling in the States and have found that even the stop signal I use, which is not generally practiced here, is well-accepted.

1. Right turn: The standard right turn signal is the right arm held out horizontally and palm of the hand held vertically. This is unambiguous. (The first time I saw the U.S. left-arm-bent-upwards signal I was waiting for vertical takeoff! This signal was evidently invented for a cabin-trapped motorist. It doesn't mean cyclists should follow suit.)
2. Left turn: The same but with the left arm held horizontally.
3. Slow, possibly preparing to stop: The "broken arm" U.S. signal is a poor substitute for the U.K. signal, IMHO, which commences by stretching out the arm as for a left turn signal but with the palm facing DOWN, and then moving it rhythmically down to the vertical position, and then back up again, repeating as necessary. I find this not only catches following drivers' attention but seems to better convey the intention. At the very least drivers will slow down to see what your next move is! It also lends itself to conveying to a following driver (by adding a backwards component to the downwards motion when necessary): "Back off and slow down you twit, can't you see I'm going to stop at this stop sign?!" - or whatever. (Again, when I first saw the U.S. equivalent, my wicked British humor interpreted it as "preparing to take a nosedive vertically downwards!")

With regards, Martin Pion. [Effective Cycling Advocate.]