Back ~20 years ago I had a small tail light to replace the reflector that had the left / right arrows. It had a wire that ran to a controller that mounted on the handlebars. Then all I had to do was slide the control from one side to the other (or the middle) to change how the lights flashed. While it was fun, I'm not sure how effective it was. Drivers don't necessarily look for turn signal on a bike, so they might just wind up looking like a generic flashing light.