Turn signal lights for bikes
I'd recommend learning to do arm signals. 

Arm signals don't run out of batteries, and are plenty visible in most cases. They're certainly bigger than the turn signal lights you could put on a bicycle (which as Moz points out in a comment, makes distinguishing the 2 turn signals a possible issue), and the distance you need to see a bike turning is a lot smaller than that which you need to see a car turning.You also have to worry about running wires from the back to front if you only have one power source (and the indicator switch isn't wireless) and worry about bags or other things obscuring the lights. Your handlebar area also becomes a bit more cluttered for a turn signal anyway, esp if you have trigger shifters. And you have to deal with the potential of the lights getting stolen. 

At night, the primary value would be moreso the extra light rather than the fact that it was a turn signal light, and you can get that just by putting on a bigger head and tail light.  

You'll get used to balancing while doing them after a few weeks of riding, typically. And in any case, you need to know how to do them when your batteries run out. 

IMO, the primary use of turn signals on a bike is less-so that you know which way a turn is happening, but moreso that the drivers know the cyclist is going to do something and be prepared for it (a sentiment echoed by one of the comments).