Originally Posted by Helmet Head
I find the most effective means to convey an intent to merge or turn in a given direction is simply to look behind me over the respective shoulder.
...
This is particularly useful when traveling fast downhill and you want to keep both hands on the wheel to brake with both hands, yet need to signal.
I do this in bumper-bumper stop/start traffic (whenever I don't feel like filtering-front), I like to have both brakes available, and sneaking a look over my shoulder looking directly AT a driver, I know he's seen me and knows I'm wanting to come in. If I'm not "in" traffic as much, or moving much slower, etc. then I'll put my arm out - this is the norm, I don't have a problem signalling. I often find I don't signal much or at all if there is no traffic around, though.
I don't use the left-arm right-turn signal, simply because I personally think it's stupid. I use my left arm (typically as I'm merging left) raised in that similar manner to wave "thanks" to a courteous driver, and I'm usually taking the lane, so my left and right arms should both be equally visible (and hopefully to MANY of the non-native drivers here, a right-arm and left-arm signal are very easy to understand and not misinterpret).
YMMV on hand signals, but I believe them to be a perfectly good solution rather than needing explicit lit signals.