Pedestrians, parked cars and a beam with no edge
Thornton Heath is densely built, and most side streets are narrow Victorian terraces with parking on both sides, leaving room for one car at a time. Driving those roads at night is close-range work: a child between two parked cars, a cyclist pulling out, a wheelie bin left half in the road. What helps there is a beam with a defined edge and a strong, even spread across the near and middle distance, which is precisely what an oxidised lens cannot deliver however good the bulb behind it is.
Match days make the point. On Crystal Palace home evenings the roads around Selhurst Park are busy with people on foot well before and after kick-off, with closures and tight parking controls in place, and Brigstock Road and the streets around the clock tower fill up with pedestrians crossing between vehicles. A yellowed lens scatters its output, softening the cut-off and sending light upwards where it dazzles rather than forwards where it would pick somebody out in good time.
The cars themselves rarely have the luxury of a driveway. Street parking means permanent exposure to sunlight, and the thin ultraviolet hard coat on the outer face of each lens is what wears through first. After that the polycarbonate oxidises, producing the brown tint, the milky haze and sometimes a network of fine surface cracks. It generally starts showing somewhere between five and eight years of age, earlier on a car that is always outside.
We check whether the damage is outside or inside the unit, mask the paint, grille and trim so no abrasive touches bodywork, wet sand through the grades, machine polish and then apply a UV-stable clear coat, because polishing alone leaves bare plastic that hazes again quickly. The beam pattern and aim get checked at the end. The run over is Thornton Road to the Lombard roundabout, then west across Mitcham Common and south into Hackbridge, about 15 minutes.