How do I drive safely in fog, and when should fog lights be on?
The Highway Code sets a clear threshold for fog lights, and using them outside it is an offence. What that threshold is, how to judge it, and the driving technique that actually reduces risk.
- Difficulty
- beginner
- Time
- 12 min
- Read
- 4 min
- Safety
- warning
Short answer
Use dipped headlights whenever visibility is reduced, and use fog lights only when visibility is seriously reduced — the Highway Code defines that as generally when you cannot see more than 100 metres. You MUST switch them off when visibility improves. The technique that matters most is slowing to a speed at which you could stop within the distance you can actually see, and resisting the urge to follow the tail lights in front.
Fog is unusual among driving hazards because the danger is not loss of grip — the road surface is often perfectly good — it is loss of information. You are being asked to make decisions on a fraction of the usual data, and the two instinctive responses both make things worse. The first is to speed up slightly to get out of it. The second is to lock onto the tail lights ahead, which feels reassuring and actually means driving at the speed of a driver you cannot see, at a distance you have not chosen, with no idea what is beyond them.
Safety
Step by step
- Turn dipped headlights on as soon as visibility drops.Rule 226 of the Highway Code says you MUST use headlights when visibility is seriously reduced. Daytime running lights are not enough, because on most cars they illuminate the front only and leave you with no rear lights at all — which is precisely the direction the danger comes from in fog.
- Use fog lights only when you genuinely cannot see 100 metres.Rule 226 permits front or rear fog lights when visibility is seriously reduced, generally taken as less than 100 metres, and Rule 236 says you MUST NOT use them otherwise and MUST switch them off when visibility improves. That is a legal requirement, not a courtesy.
- Judge 100 metres against something real.A useful calibration on a motorway or dual carriageway is the marker posts, which are spaced 100 metres apart on many roads. On other roads, roughly the length of a football pitch, or about 25 car lengths. If you can comfortably see further than that, your fog lights should be off.
- Slow to a speed you can stop within.This is the whole of fog technique in one sentence. If you can see 50 metres, you need to be able to stop in less than 50 metres. Speed limits are maximums for good conditions, not targets in fog.
- Increase the gap, and do not follow tail lights.The lights ahead tell you where a car is, not what is beyond it. Following them closely puts you in the position of reacting to their brake lights instead of the road, and it is how pile-ups form. Leave a gap you have chosen.
- Use the windscreen and demister actively.Fog is water, and it fogs the inside of the glass as well as the outside. Wipers on intermittent, demister running, and air conditioning on if you have it — it dries the air and clears the inside far faster than heat alone.
- Open a window at junctions.In thick fog you can often hear an approaching vehicle before you can see it. It costs nothing and at a rural junction it is genuinely useful.
- Switch the fog lights off the moment it clears.Rear fog lights are as bright as brake lights and mask them completely from behind, which is exactly why leaving them on is an offence. Front fog lights dazzle oncoming drivers. A useful habit is to check them every time you slow for a junction.
Common mistakes
- Leaving rear fog lights on after the fog clears — They are as bright as brake lights, so following drivers cannot tell when you are braking. It is also specifically prohibited by Rule 236.
- Using main beam in fog — The light reflects straight back off the water droplets and reduces what you can see rather than increasing it. Dipped beam is genuinely better.
If it doesn't work
Windscreen keeps fogging on the inside
Cause: Warm damp air meeting cold glass — Fix: Air conditioning on with the demister — the system dehumidifies the air, which heat alone does not. Aim air at the screen rather than at your feet, and avoid recirculation, which traps moisture in the car.
Patchy fog that clears and returns
Cause: Typical of valleys, river crossings and open country at dawn — Fix: This is where the fog light rule catches people, because visibility genuinely changes every few hundred metres. Err towards switching them off in the clear patches.
Someone behind is very close
Cause: They are using your lights as a guide — Fix: Do not speed up and do not brake sharply to make a point. Keep your own gap to the front, stay steady, and let them past when it is safe.
Cannot tell whether your fog lights are on
Cause: Symbols are similar and the switch is often on a stalk — Fix: The dashboard tells you: an amber symbol with straight lines through the beam is rear, a green one is front. Learn which is which on your car in daylight rather than in fog.
Questions people ask
Is it illegal to use fog lights when it is not foggy?
Yes. Rule 236 of 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. That wording indicates a legal requirement, and drivers do get fixed penalties for it.
Do daytime running lights count?
No. On most cars daytime running lights operate only at the front, so a car relying on them in fog has no rear lights showing at all. Switch the headlights properly on.
Should I use hazard lights in fog?
Only briefly, on a motorway or dual carriageway, to warn drivers behind of a hazard or queue ahead. Driving along with them on is not permitted and it removes your ability to indicate.