Martin, I feel, from your tone, that you are trying to have a go at me for something.
You said above that RT's point made little sense to you, I was just trying to explain that, generalisation or not, it is possible for dark clothing to allow something to remain effectively hidden, even when illuminated by your headlights.
Whilst I'm sure we always try to travel at a speed which allows us to stop in the distance that we can see to be clear, it is also possible for that distance to be under 40 feet. To use the quidance from the highway code, that would require every unlit road to be travelled at speeds of below 20mph, just on the off-chance that some unlit numpty, or cow, wants to remove himself from the gene pool.