The short answer
The Highway Code says you must use headlights when visibility is seriously reduced, which it puts at generally not being able to see for more than 100 metres. You may use front or rear fog lights as well, but you must not use them unless visibility is seriously reduced, and you must switch them off when it improves. Both are law, not advice: the Code cites the lighting regulations for each.
The reason is the second half of rule 236. Fog lights dazzle other drivers, and a rear fog light can hide your brake lights. Left on after the fog has cleared, they make you harder to read, not easier to see.
What counts as fog
The Met Office's definition is visibility below 1000 metres; in public forecasts fog generally means visibility under 180 metres, and dense fog is under 100 metres. Mist is visibility over 1,000 metres. In other words, the 100-metre headlight line in the Highway Code is roughly dense fog as the Met Office defines it.
The fog that catches drivers in autumn is radiation fog. It forms on clear, calm nights as the ground loses heat and the air just above it cools until the moisture condenses. It is most common in winter, it sits in valleys and over damp ground, and it usually lifts shortly after sunrise once the ground warms, which is why the morning commute is the problem and the drive home often is not. Before you set off, look at the Met Office warnings page; fog warnings are issued when it is expected to be dense and widespread.
The rules, one by one
| Rule | What it says | Must or should |
|---|---|---|
| 226 | Headlights on when you cannot see for more than about 100 metres | Must |
| 226 and 236 | Fog lights only when visibility is seriously reduced; off when it improves | Must |
| 234 | Check mirrors, then slow down before entering fog; expect banks of fog after a Fog sign even if the road looks clear | Should |
| 235 | Keep a safe distance; rear lights give a false sense of security; be able to stop well within what you can see | Should |
| 235 | Use wipers and demisters; watch for cars with no lights; do not speed up to escape a tailgater | Should |
| 235 | Check mirrors before slowing, and brake so your brake lights warn the driver behind | Should |
| 126 | Never closer than the full stopping distance; at least two seconds on fast roads, doubled when wet | Should |
Two points the old advice gets wrong. The Code does not say to use main beam in fog, and most drivers find it reflects straight back; dipped headlights are what rule 226 requires. Nor does the Highway Code give a fixed following time for fog. It says to be able to pull up well within the distance you can see clearly, which in dense fog is a much bigger gap than the two seconds of rule 126.
Does your car's fog light even work
Rear fog lamps are tested at the MOT, and on older cars they fail more than you might think. In tests carried out 2021 to 2025, a rear fog lamp not working was recorded as a fail in 1.1% of first tests on cars aged 15 and over, against 0.5% at 10-14, 0.2% at 6-9 and 0.1% at 3-5. A dipped-beam headlamp not working, the light rule 226 actually depends on, was recorded in 2.1% of tests at both 10-14 and 15 and over, 1.1% at 6-9 and 0.4% at 3-5.
Neither is a big share, but a rear fog lamp is the one bulb nobody checks until the morning they need it. Find the switch in your car before the fog arrives, and check the lamp comes on and, as importantly, that you know how to turn it off again; on many cars the rear fog light stays on after the ignition is cycled.
What to do before a foggy morning
- Look at the forecast the night before. Clear sky, no wind and a damp ground after rain is the recipe. Allow extra time; fog usually lifts after sunrise, so leaving later can be the safest change.
- Find the fog light switch and check both the rear lamp and the dipped beams work.
- Clear the glass inside and out and fill the washer bottle; rule 235 expects wipers and demisters in use.
- Dipped headlights on as soon as you cannot see 100 metres, fog lights only if it is worse than that, and off the moment it clears.
- Drop your speed to the distance you can see, not to the car in front, and leave a gap that does not depend on its rear lights.
- Brake early and gently so your brake lights do the warning for you.
What this does not mean
This is a plain-language summary of the Highway Code rules and the Met Office definitions as they read on 28 October 2026, not legal advice; the Code and the regulations it cites are the authority. The MOT figures describe how often two lamp items were recorded as fails across millions of tests by age; they say nothing about whether the lamps on any one car work today, which is what the switch and a walk round the car are for.
Frequently asked questions
The Highway Code says you must not use front or rear fog lights unless visibility is seriously reduced, and must switch them off when it improves, citing the lighting regulations. So yes, leaving them on in clear conditions breaks the rule, because they dazzle and can hide your brake lights.
Rule 226 says when visibility is seriously reduced, generally when you cannot see for more than 100 metres. A practical test is whether you can see the car two ahead, or the next lamp post.
The Code requires headlights, and in practice dipped beam. Main beam lights up the fog in front of you rather than the road, and dazzles oncoming drivers. Use dipped beam, and fog lights only if visibility is seriously reduced.
The Code gives no set time for fog. Rule 235 says keep a safe distance and be able to pull up well within the distance you can see clearly, and warns that rear lights give a false sense of security. If you are judging the gap by the lights ahead, you are too close.
Sources and method
The rules are quoted from the Highway Code pages on GOV.UK and the Met Office fog page as they read on 28 October 2026. The two MOT figures are national: first tests in each age band (test year minus first-used year) that recorded the item as a FAIL, divided by all first tests in the band, test years 2021 to 2025 pooled; bands hold 33,793,352 (aged 3-5), 50,103,915 (6-9), 44,980,002 (10-14) and 31,405,150 (15+) first tests.
- GOV.UK: The Highway Code, driving in adverse weather conditions (rules 226 to 237)
- GOV.UK: The Highway Code, general rules for all drivers and riders (rules 103 to 158)
- Met Office: Fog
- Met Office: UK weather warnings
- DVSA anonymised MOT results, class 4 first tests, 2021-2025
About these figures. Figures are computed by SortedCars from the DVSA anonymised MOT results dataset (class 4, first tests only, test years 2021-2025), published under the Open Government Licence v3.0. Pass rate = passes divided by first tests, with pass-after-rectification counted as a fail. No figure is shown for any group with fewer than 500 tests. Test class 4 only, first tests only; retests are excluded. Pass rates describe fleets, not individual machines: they reflect how cars were driven, maintained and used as well as how they were built, and beyond recorded mileage none of that is in the data. A well-kept example of a model with a weak average can still pass; a neglected example of a strong one can still fail. Alias spellings of a make are counted under one make name and all trims of a model under one model name, using SortedCars' grouping tables.
This article is general information drawn from published test records. It is not professional, mechanical or legal advice, and an MOT pass or a clean MOT history does not guarantee that a car is roadworthy today. If you are unsure about a car, have it inspected by a qualified mechanic. You can check any vehicle's MOT history free on the official GOV.UK MOT history service.
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