Fog lights — the 100-metre rule, heavy rain, and when you MUST switch them off

Almost every fog-light argument is really an argument about the word “fog”. The rule does not mention weather at all: it turns on how far you can see, and the Highway Code puts a number on that — 100 metres. Get the test right and the rest falls out of it. Torrential rain can qualify. A dark, drizzly evening you can still see down does not. And there is a second MUST most drivers forget, the one that says switch them off again.

You MUST use headlights when visibility is seriously reduced, generally when you cannot see for more than 100 metres (328 feet). You may also use front or rear fog lights but you MUST switch them off when visibility improves (see Rule 236).

Read that sentence slowly, because it does three separate jobs. Headlights are compulsory. Fog lights are optional — “you may also use”. And switching the fog lights off is compulsory again the moment things clear.

The whole rule, in one table

Rear fog lamps are the ones that start arguments, and the asymmetry above is why: nearly every car on the road has one, hardly anyone is taught what it is for, and it points straight into the eyes of the person behind.

“Seriously reduced visibility” is the test — fog is only the usual cause

The prohibition sits in the Road Vehicles Lighting Regulations 1989, and it is worded as a visibility condition, not a weather condition. Front and rear fog lamps get separate entries in the table of prohibited uses, and both carry the identical middle line.

Used so as to be lit at any time other than in conditions of seriously reduced visibility.

The regulations then decline to define the phrase. It appears nowhere in the interpretation table at regulation 3 — the only place “seriously reduced visibility” turns up there is inside the definitions of the lamps themselves: a front fog lamp is one “used to improve the illumination of the road in front of a motor vehicle in conditions of seriously reduced visibility”, a rear fog lamp one “used to render a vehicle more readily visible from the rear” in the same conditions. Circular — the regulations never attach a number to it.

What fills the gap is Rule 226’s gloss: generally when you cannot see for more than 100 metres. That figure is guidance rather than a statutory threshold, but it is the only number the official texts offer, and it is the number the theory test is written against. Treat it as the working definition.

A hundred metres is shorter than most people picture — two and a half intervals of the chevron markings painted on some motorways at 40-metre spacing. If you can still pick out traffic comfortably beyond that, you are not in seriously reduced visibility.

So: heavy rain and motorway spray?

Yes — when the rain or the spray genuinely takes your visibility below the line. The rule asks how far you can see, so anything that shortens that distance enough qualifies. Rule 227 explicitly warns that “the rain and spray from vehicles may make it difficult to see and be seen”, and a lorry throwing up a wall of water on a wet motorway can put you well inside 100 metres of usable vision.

That is the honest answer to the argument that fills every forum thread, and it cuts both ways.

• Standing water and dense spray on a motorway, wipers on fast, the vehicle two in front invisible. That is seriously reduced visibility. Headlights are compulsory; rear fog lamps are permitted, and are doing exactly the job the regulations describe — making you visible from behind.
• Steady rain on an A-road, wipers on intermittent, traffic clearly visible several hundred metres ahead. That is not. Headlights, yes. Fog lamps, no.
• Dark, wet and miserable, but you can see fine. Darkness is not reduced visibility. Rule 113 already requires headlights at night; it does not unlock the fog lamps.
• A heavy shower that lasts ninety seconds. Permitted while it lasts, prohibited the moment it passes — which is the part almost everyone skips.

Two things do not change with the weather. The dazzle prohibitions are separate lines in regulation 27 and apply on their own terms, so “conditions were bad” is no answer to blinding the driver behind. And in a queue that has stopped, a rear fog lamp is achieving nothing the car cannot already do — the driver two metres away can see you without it.

The second MUST: switching them off

Rule 236 is the enforcement half of the pair, and it explains itself.

You MUST NOT use front or rear fog lights unless visibility is seriously reduced (see Rule 226) as they dazzle other road users and can obscure your brake lights. You MUST switch them off when visibility improves.

Two harms, both worth knowing for the exam. Dazzle is the obvious one. The second is subtler and more dangerous: a rear fog lamp sits in the same cluster as the brake light and is bright enough to be seen through fog, so when you brake, the driver behind may not register the change. You have swapped a warning signal for a permanent glare. On some cars the two even share hardware: the MOT inspection manual notes that a proportion of the stop-lamp LEDs may extinguish when the rear fog lamp is illuminated, and that this is not to be regarded as a defect — which leaves the brake signal weaker still.

The same lighting rule appears again in the general lighting section, Rule 114, alongside the broader ban on using any lights in a way that would dazzle or cause discomfort to other road users, including pedestrians, cyclists and horse riders.

Your car will tell you the lamps are on if you look. A rear fog tell-tale on the dashboard is required wherever the rear fog lamp itself is mandatory, and an MOT tester records a missing or inoperative one as a major defect.

Headlights are the compulsory part

The pairing that catches people out is that the fog lamps are optional and the head lamps are not. Regulation 25 requires obligatory dipped-beam headlamps to be kept lit “in seriously reduced visibility” as well as during the hours of darkness, and Rule 226’s MUST is that requirement restated.

There is one narrow substitution. A car fitted with a pair of dipped-beam headlamps may run “a pair of front fog lamps which is so fitted that the outermost part of the illuminated area of each lamp in the pair is not more than 400 mm from the outer edge of the vehicle” instead, and only in seriously reduced visibility. That is a real exception, but it is a specification test most cars fail, and it never permits front fog lamps as a general alternative to headlights.

Daytime running lamps are not headlamps. Rule 226’s MUST, and regulation 25 behind it, are about obligatory dipped-beam headlamps. The regulations define a daytime running lamp as a separate device altogether, alongside the running lamp they describe as one “used to make the presence of a moving motor vehicle readily visible from the front”. Neither is a headlamp, and neither satisfies regulation 25. That is why a car can look convincingly lit from ahead in murk and show nothing at all from behind — the exact scenario Rule 235 has in mind when it tells you to beware of other drivers not using headlights. If the dashboard is bright and you have touched nothing, check.

Parked cars, and fog

Regulation 27 bans front and rear fog lamps from being lit while a vehicle is parked, with a narrow exception for the rear lamp on an emergency vehicle. What you use instead is set out in the waiting and parking rules: all vehicles MUST display parking lights when parked on a road or lay-by where the limit is over 30 mph, and Rule 251 adds the specific case.

Parking in fog. It is especially dangerous to park on the road in fog. If it is unavoidable, leave your parking lights or sidelights on.

Parking lights or sidelights — not fog lights.

What it actually costs

Rules 226 and 236 are written in capitals with a Law line underneath citing RVLR regulations 25 and 27, which makes them legal requirements rather than advice — the distinction our MUST vs should guide works through in full. Breaching a lighting regulation is an offence under section 42 of the Road Traffic Act 1988, and Schedule 2 to the Road Traffic Offenders Act 1988 sets the punishment: a summary fine, level 4 on the standard scale if committed in respect of a goods vehicle or a vehicle adapted to carry more than eight passengers, level 3 in any other case. Level 3 is £1,000; level 4 is £2,500.

The disqualification, endorsement and penalty-point columns of that entry are blank. So misuse of fog lights is a fine-only, non-endorsable offence: no points, no ban. It is still an offence, and it is still on the theory test. More of the same shape are in our guide to driving offences you might not know about.

Northern Ireland

The Northern Ireland Highway Code carries Rules 226 and 236 in the same numbering with the same 100-metre gloss and the same two obligations. Only two things differ. The Law lines cite the Northern Ireland lighting regulations — regulations 28 and 30 rather than 25 and 27 — and the Code prints its legal requirements in lower case instead of capitals, so the Law line is what marks a rule as law. Our Northern Ireland theory test guide covers the rest of the differences.

How this comes up on the theory test

Fog lights sit in two of the fourteen DVSA question-bank topics — vehicle handling, which covers driving in rain, fog, snow, dark and wind, and safety margins. The recurring wrong answers are worth naming.

“You may use front or rear fog lights when it is raining”

No. The trigger is seriously reduced visibility — generally under 100 metres — not precipitation. Rain qualifies only when it cuts your visibility that far.

“You should switch fog lights off when you leave the fog bank”

Close, but the wording is stronger than should: Rule 236 makes it a MUST, and it is tied to visibility improving rather than to any particular stretch of road.

“Rear fog lights help the driver behind judge their distance”

The opposite. They give a false impression of distance, they dazzle, and they can mask your brake lights. Rule 235 makes the related point about following a vehicle in fog: rear lights can give a false sense of security.

“In fog you should keep close to the vehicle in front so you can follow its lights”

A perennial trap. Rule 235 says keep a safe distance and be able to pull up well within the distance you can see clearly — particularly on motorways and dual carriageways, where speeds are higher. Rule 260 repeats it for motorways: increase the gap on wet or icy roads, or in fog. The arithmetic behind the gap is in stopping distances, and the lane discipline that goes with poor motorway visibility is in motorway lane discipline.

“Hazard warning lights are the right thing to use in fog”

Only in the narrow cases Rule 116 allows: stationary, to warn of a temporary obstruction, or briefly while moving on a motorway or unrestricted dual carriageway to warn following drivers of a hazard ahead. Fog on its own is not a reason to drive with them flashing.

Practise it

Read Rules 113 to 116 and 226 to 237 straight through on the Highway Code — the lighting rules and the adverse-weather rules cross-reference each other constantly, and they make far more sense together than apart. Then work the DVSA topics, check the abbreviations in the glossary, and sit a full mock theory test under the real timer. It is free to get started.

Sources

• GOV.UK — Highway Code: driving in adverse weather conditions (Rules 226, 227, 234 to 236)
• GOV.UK — Highway Code: general rules, techniques and advice for all drivers and riders — lighting requirements, Rules 113 to 116
• GOV.UK — Highway Code: waiting and parking (Rules 249 and 251)
• GOV.UK — Highway Code: motorways (Rule 260)
• GOV.UK — Know your traffic signs: motorway signs, signals and road markings — chevron vehicle-spacing markings
• legislation.gov.uk — Road Vehicles Lighting Regulations 1989, regulation 27 (restrictions on the use of lamps)
• legislation.gov.uk — Road Vehicles Lighting Regulations 1989, regulation 25 (use of headlamps and front fog lamps)
• legislation.gov.uk — Road Vehicles Lighting Regulations 1989, regulation 3 (interpretation — front fog lamp, rear fog lamp, running lamp, daytime running lamp)
• GOV.UK — MOT inspection manual: cars and passenger vehicles, lamps, reflectors and electrical equipment (sections 4.3, 4.5 and 4.9)
• legislation.gov.uk — Road Traffic Act 1988, section 42 (breach of construction and use requirements)
• legislation.gov.uk — Road Traffic Offenders Act 1988, Schedule 2 (prosecution and punishment of offences)
• legislation.gov.uk — Sentencing Act 2020, section 122 (the standard scale of fines)
• nidirect — Driving in adverse weather conditions (226 to 237) — Northern Ireland

Highway Code excerpts are reproduced under the Open Government Licence v3.0.