Street-parked all week, working hardest by the Common
Almost everything about Streatham parking is bad news for a headlamp lens. Controlled zones have been spreading across the area, many side streets are narrow Victorian and Edwardian terraces with cars parked on both sides, and very few of those cars have anywhere covered to live. A lens takes ultraviolet light all day whether the car is used or not, and the thin factory hard coat is what pays for it. Once that coating thins and fails, the polycarbonate underneath oxidises to the familiar yellow-brown haze.
The driving then exposes the result. Streatham High Road and Streatham Hill are bus-heavy red routes with a 20mph limit and frequent crossings, so they are well lit and slow, but Streatham Common North along the northern edge of the Common, the roads around The Rookery and Norwood Grove, and the stretches bordering Tooting Bec Common are considerably darker. Those are the roads where a clean dipped beam earns its keep, and where a scattered one makes picking out a pedestrian crossing between parked cars harder than it should be.
Fitting brighter bulbs is the common first response and the wrong one. Light has to get out through the lens, so a brighter source behind a degraded surface produces more scatter rather than more usable range, and it increases the glare for oncoming traffic on a narrow terraced street. Clearing the lens restores the shape of the beam, which is what actually determines how far down the road you can see and how little you annoy everyone else.
We look at the unit first, because condensation inside, a crack through the lens or a reflector that has burnt or peeled all mean a new unit rather than a polish. If the surface is the problem, we mask the paintwork and trim, wet sand through the grades, machine polish, apply a UV-stable clear coat and check the beam pattern. The run down is Mitcham Lane and Streatham Road through Mitcham, about 20 minutes, so most people make a morning of it.