Why a clean windshield is a safety issue

What glare is actually showing you

Clean glass lets light pass straight through. Dirty glass scatters it. That scattering is why a windshield that seems clear on an overcast afternoon suddenly glows white when you head west into a low sun, or when a truck with bright headlights comes over the hill. The dirt was always there. The angle of the light just made it visible.

In London that low sun shows up on the morning and evening commute for much of the fall and winter, exactly when the roads are wet and salty. It is the worst combination: a film on the outside from road spray, and the light coming in at the angle that shows it most.

The outside: salt, wiper streaks and bugs

In winter, spray from other vehicles leaves a salty haze that the wipers smear rather than remove. Washer fluid helps, but only if you use a winter-rated fluid and keep the reservoir topped up. Running out on the 402 in a slush storm is a real hazard, so keep a spare jug in the trunk from November on.

Worn wiper blades leave streaks and lines that catch light at night. If your blades chatter, skip, or leave a band of water across the middle, replace them. Many drivers swap blades in the fall and again in spring. In summer, bug remains are the main problem: they dry hard and wipers mostly spread them. Soak them with washer fluid or a bug remover before letting the wipers run.

The inside is usually the dirtier side

The haze on the inside of the windshield comes from several places: plastics in the dashboard giving off vapours on hot days, breath, heater air carrying dust, vaping or smoking, and fingerprints from wiping fog with your hand. It builds slowly, so you rarely notice it until night driving gets uncomfortable.

A car wash cleans the outside only, so the inside is a job for you. It takes about ten minutes.

• Use two clean microfibre cloths: one damp with glass cleaner, one dry.
• Spray the cloth, not the glass, so cleaner does not drip onto the dashboard.
• Wipe in one direction across the glass, then the other direction with the dry cloth.
• Use the back of your hand to push the cloth into the bottom corner where the glass meets the dash.
• Finish with the side windows and the mirror, which collect the same film.

Winter habits that help

Clear all the snow and ice before driving, including the roof, so it does not slide down over the glass when you brake. Let the defroster do its job instead of scraping hard at a half-thawed windshield, and do not pour hot water on frozen glass: the temperature shock can crack it, especially if there is already a chip.

Speaking of chips: a small stone chip is cheap to repair while it is small. Freeze and thaw cycles, and the defroster blowing warm air onto cold glass, can turn a chip into a crack that runs across the windshield. Get chips looked at before the first cold snap.

How often to clean the glass

The outside gets cleaned every time you wash the car, which in a salty winter might be every week or two. The inside needs attention far less often, once a month or so, and more often if anyone smokes or vapes in the car. If night driving starts to feel tiring or oncoming headlights seem to flare, clean the inside before you blame your eyes.