The glare that blinds you in September is mostly on the inside of your windscreen
For two weeks either side of the equinox the sun rises at true east and sets at true west, and a great deal of Europe drives to work along that exact line.
It is the one date in the calendar when the geometry is the same everywhere on the planet at once, and the road network was never laid out to agree with it, yet enough of it runs east to west that the effect lands on millions of commutes at the same time. The morning drives into a sunrise, the evening drives into a sunset, and both sit at an elevation low enough to arrive exactly where the windscreen is rather than where the roof is. Great Britain is one of the few countries in Europe that asks its police to record a cause this specific, and across the five years to 2021 dazzling sun was logged as a contributory factor in collisions that killed 28 people and injured 2,876 every year on average. What makes those numbers worth reading twice is who they land on, because 74 percent of the deaths were pedestrians, cyclists and motorcyclists, road users who between them account for roughly 2 percent of the distance driven on those roads. The driver is dazzled and somebody outside the car pays for it. And since the sun is precisely as bright for every driver on that road, the thing that decides whether you squint or lose the road entirely is not the weather. It is the half millimetre of glass, film and grime sitting between the sun and your eye.
The haze on the inside of your windscreen has an industry standard written against it
The car industry has a name for it and a test method to match. Under DIN 75201, the fogging standard also published as ISO 6452, a sample of interior trim is heated to 100 degrees Celsius for sixteen hours while a glass plate held at 21 degrees sits above it, and the laboratory then measures what has condensed on that plate, either by reading its gloss at a 60 degree angle or by weighing the deposit. The standard exists because dashboards, adhesives, foams and seat materials give off semi volatile compounds for years, and a parked car in the sun is a warm chamber with one cold surface in it, which is the windscreen. The film that lands there is genuinely invisible when you look through it at a dark road at night, because scattering needs something to scatter. Point the same glass at a sun sitting five degrees above the horizon and every particle in that film lights up at once, the contrast in front of you collapses, and a screen you would have sworn was clean turns into a lit panel. That is why the glare seems to arrive out of nowhere in September on a car you washed in July: you washed the outside.
Your sun visor was designed for a sun that is higher than this one
A visor hinges down from the header rail and covers the top of the screen, which is the correct answer for a summer sun overhead and the wrong answer for an equinox sun at eye level. Pulled far enough down to block a low sun it also blocks the road, and most drivers end up holding it at a compromise angle that blocks neither. Two things help more than the visor does. The first is sliding it out of its clip and swinging it to the side window, which is the position almost nobody uses and the one that matters on a road curving across the light. The second is remembering that the sun also arrives from behind, and that most self dimming rear view mirrors will not help you there, because they carry a forward facing ambient sensor whose job is to inhibit dimming in daylight so the mirror stays usable. In bright low sun the mirror stays bright by design. That is worth knowing alongside the fact that your door mirrors are probably aimed at your own bodywork, because a mirror full of your own paint is a mirror full of reflected sun.
Sunglasses carry a category number, and one category is marked as unfit to drive in
Every pair of sunglasses sold in Europe is classified under EN ISO 12312-1 on a scale from 0 to 4, and the number is printed on the arm or the label rather than advertised on the price tag. Category 3 lenses pass between 8 and 18 percent of visible light and are the driving category, the one you want in the glovebox from September onwards. Category 4 lenses pass 3 to 8 percent, are built for glaciers and high altitude, must be marked as unsuitable for driving, and are explicitly forbidden at the wheel in France. Nothing above category 0 belongs on your face in a tunnel or after dark, which is the trap on an autumn evening when the dazzle stops and the light goes within a few minutes of each other. Polarised lenses are a separate decision and a genuinely good one for the horizontal glare coming off wet tarmac and off your own bonnet, with one caveat worth testing before you rely on them: polarisation can black out an LCD instrument panel or a head up display at certain head angles, and you want to discover that in a car park rather than at 120 km/h.
When the sun is behind you, you are the thing nobody can see
Half of the equinox problem is aimed at other people. Drive west into the evening sun and you are dazzled. Drive east with it behind you and you are a silhouette against a bright sky for everybody ahead, at the exact moment your daytime running lights are pointing away from the traffic that needs to see you, since daytime running lights are a front only device and light nothing at the back. Dipped beam costs nothing and puts your tail lamps on. The second half is what the dazzled drivers around you are about to do, because a driver who sees a queue late does not brake progressively, and the whole reason the industry fitted brake assist is that almost nobody presses hard enough when it matters. Add distance in the dazzle band and read the traffic three cars ahead, because brake lights seen against a low sun are the one signal the glare erases first.
What to actually do
Twenty minutes and one pair of sunglasses covers almost all of this, and the single highest value action on the list is the one nobody does.
Clean the inside of the windscreen, and treat it as a separate job from the outside rather than an afterthought to it.
Use two cloths, one lightly damp with a glass cleaner and one dry, and finish with the dry one before the first is done evaporating, which is what removes the streaks that cause the next dazzle.
Reach the bottom 15 centimetres where the glass meets the dashboard. That strip is the hardest to reach, collects the most deposit and sits directly in the low sun line.
Keep silicone dashboard shine products away from the screen entirely. They are the exact chemistry the fogging test was written to measure.
Replace any wiper blade that leaves a visible arc. That arc is a smear you never notice at night and a white band at sunrise.
Fill the washer bottle with proper screen wash rather than water, because the film outside is insect protein and road film, and water alone spreads both.
Check the category number on your sunglasses. Category 3 is the driving lens, category 4 is marked as unsuitable for driving and is illegal at the wheel in France.
Keep them within reach of the driver's seat, not in the boot. A pair you have to stop for is a pair you end up driving without.
Switch to dipped beam whenever the low sun is behind you, and add following distance in both directions until the sun clears the roofline.
Frequently asked questions
Why does the glare get worse right after I wash the car?
Because a car wash cleans one side of the glass and leaves the other one alone, and the inside is where the scattering film lives. There is a second reason on cars that go through a brush wash regularly, which is that fine scratches in the outer surface do exactly what the internal film does, scatter low angle light rather than pass it. Nothing removes those, but the film you can remove is doing most of the work, so the inside clean is where the improvement comes from.
What actually causes the haze on the inside of the glass?
Outgassing from the cabin itself. Dashboard plastics, adhesives, foams, carpets and leather treatments release semi volatile compounds slowly and much faster when hot, and in a parked car the windscreen is the coldest large surface for them to condense on. The industry tests for it under DIN 75201, which is why interior materials carry a fogging rating at all, but no material scores zero and no car is exempt. Tobacco smoke, air fresheners and silicone trim dressings all add to it, and a car parked outside through a hot summer will have built the thickest film of the year by about now.
Which sunglasses are right for driving?
Category 3 under EN ISO 12312-1, which transmits 8 to 18 percent of visible light and is the darkest lens the standard considers appropriate for general road use. Category 2 is a reasonable choice if you drive a lot of tunnels or move in and out of woodland. Category 4 is the one to avoid, at 3 to 8 percent transmission it is built for glaciers, it must be marked as unsuitable for driving, and France bans it at the wheel outright. Check the arm of the frame, since the number is a labelling requirement rather than a marketing feature and cheap glasses carry it just as readable as expensive ones.
Are polarised lenses better in a car?
For daytime glare, usually yes. Polarisation targets light that has been reflected off a horizontal surface, which is precisely the sheet of white you get from wet tarmac, from a low sun on a damp road and from your own bonnet. The caveat is what it does to screens: LCD instrument clusters and some head up displays are themselves polarised, so at certain head angles they dim or vanish completely. Test yours parked before an autumn trip, because a dark instrument panel is a poor trade for a clearer road.
Should I put my headlights on if the sun is behind me?
Yes, and dipped beam rather than daytime running lights. A low sun behind you turns your car into a silhouette for every driver ahead, and it dazzles the ones looking at you in their mirrors, so you want to be the shape with two red lamps on it rather than the shape without. Daytime running lights do not do that, because on almost every car they are a front only system and leave the tail lamps unlit. One movement of the light switch fixes it, and it costs you nothing at all.
