Left Knutsford services at quarter past four one November morning. Dense fog across the whole M6 from Junction 18 southbound — the kind where you can see the lights of the truck in front but not much beyond them. I had everything on: full headlights, rear fog, front fogs. Thought I was doing the right thing.
By Junction 16 the fog had more or less cleared. I was still running with everything on. Didn't think anything of it until the car behind me flashed his headlights three times and pulled alongside to gesture through his window. Took me a moment to figure out what his problem was.
Then I realised: front fog lights blazing in clear conditions. Not just unnecessary — an offence. Specifically, the offence of "using a front fog light other than in seriously reduced visibility conditions," under Regulation 27 of the Road Vehicles Lighting Regulations 1989. I switched them off and felt like an idiot for the next thirty miles.
The fog situation in an HGV is actually more complicated than most drivers think. There are two separate rules — one about visibility thresholds that triggers certain lights, and one about which lights can be used in which conditions. Getting them confused costs you either a ticket or a dangerous situation. Sometimes both.
The 100-metre rule
Rule 226 of the Highway Code is the visibility trigger for headlights. The wording: you must use headlights when you cannot see for more than 100 metres — about 328 feet. That's the visibility point at which main beam or dipped headlights become mandatory under highway code, not optional.
In practical terms, 100 metres is short. It's less than the length of a football pitch. In real fog, it's not hard to be below that threshold. On a motorway doing 50mph, stopping distance at that speed on dry roads is around 53 metres — less than your visibility. On wet roads or in fog, the stopping distance increases while your visibility shrinks. The 100-metre rule is a floor, not a target.
For HGV drivers running at 56mph on the limiter, the stopping distance matters more than it does for a car. Loaded, on a damp surface, in fog: the gap you need to stop is larger than the gap you can see. That's the risk you're managing every time you adjust speed in fog, and the 100-metre headlight trigger is the point at which the law says you should at minimum be running full lights.
Sidelights only below 100 metres visibility? Not compliant. Running lights without dipped headlights? Not compliant. The Highway Code is clear, and Police can and do enforce it.
Rear fog lights: when to use them, when to switch them off
Rule 236 of the Highway Code is the fog light rule. The language is firm: you MUST NOT use front or rear fog lights unless visibility is seriously reduced. And — this is the bit most drivers ignore — you MUST switch them off when visibility improves, because they dazzle other drivers.
The reason rear fog lights matter so much for HGV drivers: they're significantly brighter than tail lights. That's their purpose — to be seen in dense fog. But that same brightness, when the fog has cleared, causes serious dazzle to the driver behind you. Following an artic with rear fog lights blazing on a clear motorway at night is unpleasant at best and dangerous at worst. The driver behind can't see anything except the rear of your trailer.
"Seriously reduced visibility" isn't defined in specific metres in the Highway Code. The 100 metres in Rule 226 is the headlight trigger — a separate rule. Fog lights are for more severe conditions than that. Generally accepted as conditions where fog significantly impairs visibility and affects safe driving, not just any fog.
The practical test: if you can comfortably see the lights of the vehicle ahead and behind clearly enough to judge their distance and speed, that's probably not "seriously reduced visibility" in the fog-light sense. If you're straining to see brake lights ahead and can't judge how far away the next vehicle is, that's the territory where rear fog lights are appropriate.
Front fog lights: the offence you might not know about
This one genuinely catches people out. Front fog lights are mounted low — below the headlight level — and are designed to illuminate the road surface under a fog bank rather than projecting forward. That works in actual fog. In clear conditions, they dazzle oncoming traffic significantly because they're pointing toward the road rather than along it.
The Road Vehicles Lighting Regulations 1989, Regulation 27, prohibits using front fog lights other than in conditions of "seriously reduced visibility." The same threshold as rear fog lights. The same requirement to switch them off when conditions improve.
Unlike rear fog lights, front fog lights are optional equipment on most vehicles. A lot of trucks don't have them. Some do — often fitted as standard on modern Scanias and Volvos and DAF XFs, sometimes visible as a separate lower light cluster either side of the main bumper. If your vehicle has them and you're using them on a clear road, that's an offence. CU90 — breach of requirements as to lights — typically two or three points and a fine, though it's relatively rarely prosecuted unless there's an accident involved.
More commonly, officers mention it at a stop. "Did you know your front fogs are on?" — and you've already made a bad impression before the rest of the check has started.
The specific problem with fog on the motorway
Fog tends to come in banks on motorways. You can drive into it without warning — particularly in November and December on flat sections of the M6, M1, and A1(M). The fog might be dense for two or three junctions and then clear completely. Then appear again two miles later.
The temptation is to set everything up for the worst of it and leave it there. Full headlights, rear fogs, front fogs if you have them, and not touch any of it until you're parked up. That approach works fine in the fog bank. It becomes an offence as soon as the fog clears and you haven't switched the fogs off.
The discipline — and it is a discipline, because in the dark at 4am it's not instinctive — is to actively adjust as conditions change. Drop the rear fogs when the fog clears. Switch off the front fogs. Keep the dipped headlights on (those stay on regardless). Then put the fogs back on as required when the next bank hits.
Smart motorway variable message signs on fog-prone sections will often tell you exactly what the visibility is and what to do. Mandatory speed reductions on smart motorways in fog are enforceable by camera. Don't assume the matrix's 40mph is advisory because you've driven that stretch a hundred times without seeing DVSA enforce it. It isn't advisory.
Visibility and safe stopping distance
The Highway Code's two-second rule in normal conditions becomes almost meaningless in fog. The guidance is to keep far enough back that you can stop in the distance you can see clearly. In genuine dense fog, that means very slow speeds and large gaps.
For a loaded HGV, stopping distance at 30mph in wet conditions is approximately 75 metres. In fog, with reduced tyre grip on a damp surface, add more. If your visibility is 80 metres, 30mph is already marginal. These numbers aren't comfort reading — they're why multi-vehicle pile-ups on foggy motorways tend to involve HGVs.
The legal minimums — headlights at 100 metres, rear fogs in seriously reduced visibility — are floors, not targets. If visibility is seriously reduced, the safe speed might be 20mph on a motorway. That's uncomfortable. It's also right.
Practical checklist for fog conditions
Before leaving in fog or at night in autumn/winter conditions:
- Headlights on. Dipped, unless oncoming traffic requires otherwise.
- Check rear fog light switch position — is it on or off? Know where it is before you need it.
- Know whether your vehicle has front fog lights and where the switch is.
- Plan to actively adjust both fog lights as conditions change throughout the run.
When visibility clears after a fog bank: rear fogs off. Front fogs off. Job done. When it closes in again: switch them back on. It takes a second. The alternative is driving the next 40 miles dazzling everyone behind you and wearing a potential CU90 endorsement.
If you're tracking hours and planning runs in autumn and winter, ShiftOwt helps with the compliance planning side — EU 561 and WTD tracking automatically, so you're not adding mental load to a run that's already complicated by low visibility. £5.99 a month for drivers.
Speed in fog: the limiter isn't your protection
The speed limiter on a goods vehicle over 3.5 tonnes is set at 56mph. At 56mph on dry roads, your stopping distance is approximately 73 metres — 40 metres thinking time plus 33 metres braking. On wet roads, add about 50%. In fog with reduced visibility, your ability to see brake lights ahead may be less than 73 metres.
The limiter doesn't know what the weather is. It doesn't check visibility. It caps your speed at 56mph regardless of whether 56mph is safe in the conditions. In genuine dense fog on a motorway, 56mph is not necessarily safe — and the Highway Code's guidance to "be able to pull up well within the distance you can see clearly" would suggest speeds significantly lower than the limiter in severe fog.
Variable speed limits on smart motorways are specifically set to address this. A mandatory 40mph during fog isn't advisory — it's enforceable by camera and it's set because enforcement agencies and Highways England (National Highways, technically) have calculated that 40mph is the appropriate speed for the conditions being signalled. Driving at 56mph through a mandatory 40 section in fog is both a speeding offence and, if anything goes wrong, evidence of driving without due care.
I've been in fog on the M6 near Knutsford where the matrix was running 40mph and the cars were doing 70 around me. The temptation to speed up because everyone else is going faster is real. Resist it. The limiter doesn't protect you from the consequences of the collision that happens when the driver in front brakes at 70 in 50-metre visibility.
September to November: when fog gets serious
The worst fog conditions on motorways in Britain tend to come in the autumn — September, October, November. Ground fog forms overnight as warm ground meets cold air, particularly in the flat sections of the Midlands and along river valleys. The M1 through Northamptonshire, the M6 around Staffordshire, the A1 through Lincolnshire — all notorious for dense ground fog that forms fast and can be extremely patchy.
The good news is that autumn fog is often gone by mid-morning. The bad news is that for HGV drivers doing early morning trunk runs — out of depots at 4am, 5am, hitting the motorway before dawn — that's exactly when the fog is at its densest. You drive into conditions you can't fully assess before you leave the yard. The checks you do before departure can't tell you what the M1 at Junction 15 looks like at 5:30am.
The practical response is to slow down when you encounter it, switch on appropriate lights immediately, and not assume the fog is shallow or will clear in the next mile. It might be one junction or it might be 20 miles. Drive for what you can see, not for what you hope is there.
And switch the front fogs off when it clears. Still the bit most drivers don't do.