Speaking of LEDs, very few setups are bright enough to even compare with 21/23W incandescent, and those are very expensive. Not only will you make your "turn signals" practically invisible, bur you will also considerably diminish the light output of your brake/running light.
I don't want anyone to realize how stupid an idea this is after they are rear-ended or cut off by an inattentive cager. Having a setup like this can make it very hard to get compensation for your damages/injuries.