Quick answer: Traffic film is an oily bonded grime — unburnt fuel, brake dust and tyre particles — so it needs a two-stage clean: first a degreasing pass (warm water with a small squirt of washing-up liquid, or a dedicated TFR at car-wash dilution) to break the oil, then a residue-free pass (vinegar solution or purified water) so the glass isn't left sticky. One-stage cleaning either smears the oil around or leaves soap film that re-attracts the next week's grime.
If your home faces a busy road, you know the haze: a uniform grey dullness on the road-facing glass that's back within days of cleaning, slightly greasy when you wipe a finger through it. That's traffic film, and it behaves differently from ordinary dust — it's oil-bonded to the glass, which is why a normal spray-and-wipe just pushes it around in smears.
What you're actually cleaning off
Traffic film is a cocktail of unburnt hydrocarbons from exhausts, brake-pad dust, tyre wear particles and road grime, carried on air currents and bonded to surfaces by its own oil content. Homes within about 100 metres of a main road get the heaviest coating, and elevations facing the traffic take almost all of it — check your side windows for the contrast. It's the same film that turns white cars grey, and car detailers treat it as its own category of dirt for a reason.
Stage one: break the oil
Warm water with a small squirt of washing-up liquid, applied with a microfibre pad and a bit of pressure, emulsifies the film. For heavy build-up, a proper traffic film remover (TFR) from a motor factor, diluted as per the label, is faster. Work it across the pane and you'll see the water sheet differently as the oil breaks.
Rinse thoroughly. At this point the film is gone — but your glass is wearing a soap layer instead.
Stage two: strip the residue
Soap residue is flypaper for the next week of traffic grime — skip this stage and your windows will re-haze faster than before, which is the trap most roadside households are stuck in. Go over the pane again with 1:4 white vinegar and water, or plain purified water, and dry with a clean microfibre. Now the glass is actually bare. (This residue chemistry is also why windows streak.)
The frequency problem (and the honest fix)
Here's the part no cleaning method solves: the road refills the film continuously. Roadside glass needs cleaning three or four times as often as a quiet cul-de-sac's — and that's exactly the scenario where paying a window cleaner's UK round rates monthly gets expensive, and where doing it by hand gets abandoned.
This is the textbook use-case for automating: a robot window cleaner makes the marginal cost of a clean roughly zero, so the road-facing glass can get a weekly pass without you being involved beyond placing the robot and anchoring the rope. The CleanBot R1's microfibre pads handle light re-hazing on their own; give it the two-stage treatment seasonally and let the robot hold the line in between — and wash the pads more often than a countryside user would, because they'll load up with the oily grey to prove it.
FAQ
Does rain wash traffic film off?
No — the oil content repels water. Rain wets it, redistributes it and dries it into streaky runs, which is why roadside windows look worst after a shower.
Will vinegar alone remove it?
Light, fresh film, sometimes. Established film needs the degreasing stage first: acid deals with minerals, detergent deals with oil, and traffic film is oil.
Is it worth a hydrophobic coating?
Rain-repellent coatings help water sheet off but don't stop oily particulates bonding. They're a nice-to-have, not a fix — frequency is the fix.
If the road-facing glass has made you give up on cleaning windows altogether, start with the full list of re-soiling accelerants — traffic film is usually only one of the culprits.