I have a quite bright front light (a Cygolite Night Rover NiMH xtra) - 16W halogen, handlebar mount. It is certainly more than bright enough to be seen by cars from a far distance and to see the road at 20mph.
But I still find that compared to day riding, cars are left turning right into my path of travel - as if they don't see me, so far I have been able to avoid being hit (hard braking, swerving, etc.), but there are too many close calls. When this happen I am not hugging the right side, but in the center of the lane. In fact I've never had such close calls in daylight.
I don't think it is a raw visibiliy issue, but that the car drivers just don't know how fast I am going or how far away I am. But then again my wife and I have tested (she in car) and she says I look like a motorcycle - so why should cars think I am going slow?
I have heard that a helmet mounted front light in combination with bar light, may help with this distance perception - I really don't need one for seeing and don't want the expense - so perhaps a small LED white light may do the job (like those $10 cateye ones that are on a strap to attach anywhere)
What do others think?