It has been done many times over the years and the general consensus has always been:

1. The seperation on the typical bike is not sufficent to provide adequate visual signal.

2. Motorists are not accustomed to looking for turn signals on a bike.

3. The complexity/weight trade off is not favorable.

4. Hand signals are a viable system for cyclists.

Amazingly, a search on Google for "turn signal lights bicycle" (with out the quotes) turns up several sources for the signals. Google is your friend.

Paul