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.

In particular, when merging left, all it takes to "signal" to motorists behind you that you want to do that is often to simply look behind over your left shoulder for a second or so. Of course, look ahead first and make sure it's safe to look away for as long as you're going to look behind...

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.

Serge