Across the commons after dark on a scattered beam
Most Tooting driving happens under street lighting at 20mph, because Wandsworth has a 20mph limit on its residential streets and most of its other roads, and the Broadway junction where the A24 meets the A217 is busy with buses and pedestrians for much of the day. None of that tests a headlamp. Trinity Road heading north towards Wandsworth Common, and Tooting Bec Road crossing the commons past the Lido, are a different proposition after dark: more open, less frontage spilling light, and plenty of people on foot and on bikes.
Shift patterns matter here too. St George's Hospital on Blackshaw Road is one of the largest teaching hospitals in the country and a major employer, and Springfield University Hospital brings more traffic into Upper Tooting, so a lot of local driving happens at hours when the headlamps are doing all the work. If your lenses have gone amber and milky, you are making those journeys with a soft, scattered beam and significantly less usable light than the car was designed to produce.
Street parking is the cause. With controlled zones around the Broadway and eight-hour bays near the hospital, cars here live outdoors and often stand still for days, taking sun on the same side throughout. The factory ultraviolet hard coat on the outer lens wears thin, the polycarbonate oxidises, and you get the brown tint and the haze. Low mileage does not protect against it at all, which is why we see plenty of lightly used Tooting cars with badly degraded lamps.
If the damage is on the surface we can machine it back: masking, wet sanding through progressively finer grades with plenty of water, machine polishing and a clear coat carrying ultraviolet absorbers, followed by a beam pattern and aim check. The drive over is Mitcham Road east through Mitcham and south past Mitcham Junction, about 18 minutes. Given the distance, most people leave the car for the morning so the coating can cure properly.