What is the panels perception of the proportion of motorised vehicles with defective lights of some sort, excluding number plate lights ( I don't count those ). My view from my commute is that it is overwhemingly cars at fault and is at something like one in ten vehicles with some sort of lighting defect.
It is hard to calculate (I've counted many times) in terms of getting fair samples and at any one time you can only see part of a car and part of it's array of lights, but I say the figure could well be as high as 20%. This would include all lights and however major/minor (might even be higher than 20%). For example one year my dad's car failed it's MOT on one indicator not being orange enough in colour, since then I've started to notice how many other cars out there might have this particular problem (and a small amount of danger involved).
The ones that worry you are where you see the same car with defective lights not once but multiple times over a period of time.