what percentage of accident are caused by collision from the rear or side on?
in my experience it's usually someone getting in the way in front, or plain and simple not taking me into account when they carried out their manouvre
a decent bright lights that's big enough is enough at the rear imho, spend money on something proper big and bright on the front and, deep breath, something high viz/reflective on your body, the reflective bands I see on riders' ankles are very good
and for people with drops, the bar end lights really catch the eye