If you look at motorcycles, there is something like 50 cm (18-20 inches) between the centre line and each of the turn signals. To get something where both directions are easily identified, you would need to get both flashers ON dimly and the one which actually flashes go bright. Even with LEDs, that's a lot of battery power and a lot of wiring needed.

I think that with proper on-bike training and a well-fitted bicycle, one is able to signal in most situations with one's arm. BTW, I operate my front brake with the right hand and find it quite easy to brake and signal at the same time, even (especially) on a bike with drop bars. At night, a couple of reflective bands on the wrists should solve the problem of nighttime visibility.

Regards,