Fog Lights: When to Use Them and the Rules in the UK

Written by Petro Olekhnovych, mobile mechanic in Aylesham, Kent

What the Highway Code says

Two rules cover fog lights. Rule 226 says you must use headlights when visibility is seriously reduced, generally when you cannot see more than 100 metres (about the length of a football pitch), and that you may also use front or rear fog lights but must switch them off when visibility improves. Rule 236 says you must not use fog lights unless visibility is seriously reduced, because they dazzle other road users and can obscure your brake lights.

So fog lights are a tool for real fog, not a style choice for a dark evening or light drizzle.

A simple way to judge 100 metres

• On a motorway or dual carriageway, the marker posts on the verge are roughly 100 metres apart. If you can't see the next one, visibility is seriously reduced.
• On country lanes, think about the distance between two lamp posts or telegraph poles a few apart. If the car in front's lights vanish into the fog at that distance, it is time.
• When fog is patchy, as it often is in the valleys round Barham, Bridge and the Elham Valley, be ready to switch them on and off again as you go.

Rear fog lights: when they help and when they hurt

The rear fog light is a very bright red lamp, usually on the offside (driver's side) or in the centre. In thick fog it is what lets a driver behind see you in time. In clear conditions it causes real problems:

• It dazzles the driver behind, especially in rain where the light scatters.
• It is as bright as, or brighter than, your brake lights, so the driver behind may not notice when you brake.

Front fog lights are lower and wider than headlights. They light the road edge and the verge in fog without the glare you get from main beam bouncing back off the fog. On a clear night they add little and can dazzle oncoming drivers.

Don't rely on automatic lights or DRLs

Many modern cars have automatic headlights that switch on with low light levels. In daytime fog it can still be bright enough that they don't come on. Daytime running lights are often front only, so from behind you may have no lights at all. In fog, select dipped headlights manually, and add fog lights if visibility is below about 100 metres.

Keep your lights clean too. Road salt and grime from the A2 and M20 can cut light output badly in winter. Our Kent winter roads guide covers this and more.

Fog lights and the MOT

For most cars first used from 1 April 1980, at least one rear fog light must be fitted and working, and the warning light (tell-tale) on the dashboard must work. It must be on the offside or centre of the car, securely mounted, the right colour and not badly damaged. A blown bulb or a failed switch is a common and avoidable fail, which is why it features in our list of common MOT failures.

Front fog lights are not checked at the MOT, but if they are fitted they should still work and be aimed properly so you don't dazzle people.

Quick checks you can do at home

1. Switch on dipped headlights, then the rear fog light, and walk round the car. A shop window or garage door reflection helps if you're on your own.
2. Check the fog light symbol lights up on the dashboard.
3. Look for water inside the lamp lens. Condensation that never clears suggests a cracked lens or failed seal.
4. Check the front fog lights at the same time, along with all the other bulbs. See our guide to headlights and bulbs on dark evenings.

How we can help

If a fog light or its warning lamp isn't working, we can check the bulb, wiring, earth and switch at your home or workplace, and include all the lights in a pre-MOT check. We don't carry out MOT tests ourselves, but we help you turn up ready. Labour is £30 to £50 per hour depending on the job, and we quote before any work starts. Message your reg and postcode on WhatsApp 07563 350335 or use our contact page.

Frequently asked questions

When should I turn my fog lights on in the UK?

When visibility is seriously reduced, generally when you cannot see more than 100 metres. Switch them off as soon as visibility improves, because they dazzle other drivers and can hide your brake lights.

Is it illegal to drive with fog lights on when it isn't foggy?

The Highway Code says you must not use front or rear fog lights unless visibility is seriously reduced, and you must switch them off when it improves. Using them when it is clear can lead to a fine.

Will a broken rear fog light fail an MOT?

On most cars, yes. Cars first used from April 1980 must have a working rear fog light, and its warning light inside the car must work too. Front fog lights are not part of the MOT.

Are daytime running lights enough in fog?

No. Many cars don't light the rear lamps with daytime running lights, so you can be almost invisible from behind. Switch to dipped headlights in fog and poor light.

Sources

• The Highway Code: driving in adverse weather conditions (rules 226 to 237)
• MOT inspection manual for cars and light commercial vehicles