Here is a product that some would consider a useful safety addition for bicycles. What they are is front and rear indicator lights that let cars know which way you are going. They are wireless and they run on 2AAs each, two in the front and two in the rear. I would hazard a guess that an angry glare and a hand signal would have much more impact than Bicygnals, but I could be wrong. I have done a lot of cycling and I do mean a lot.
My mountain bike was my primary means of transportation from the age of twenty-one until I finally weakened and bought a car at age twenty-seven. In that time I encountered just about every sort of erratic driver. I got run off the road, crashed into curbs and lost a lot of skin. The thing is, not one of those times would bicygnals have saved me. When crossing roads or making turns, I always assumed that drivers would not see me. The times that I had been run off the road were late at night when drivers were most likely drunk. I found that the best thing to do about them was to mount a bright LED on the back of my bike so that they could see me when approaching me from behind. Bicygnals just look like extra unnecessary weight. Cars are not really looking for signals on bicycles, accidents are occurring because drivers are not seeing cyclists. If Bicygnals help riders be seen then I am all for them. I, however, will be sticking to my blinding, seat-post mounted LED.
Price: About $83 from Halfords in the UK