The theory being that the eye detects objects by contrast against the background, so to highlight objects they must be brightly lit or completely devoid of light. Therefore, the RAF paints some of its aircraft black and leaves the headlights on if poss. Hi Viz jackets are just more PC junk; if you stand against a similar contrast background there is no benefit at all. But I do wear mine where the risk is greatest, the walk from the carpark in the dark, where drivers are not required to have a special permit. I heard the LGW are employing sharp-shooters on the tower to pick off the guilty!