Guide To Driving In Fog

Published 23 January 2017

Fog creates dangerous driving conditions:

• It reduces visibility.
• Makes it hard to spot hazards.
• Makes it difficult to judge distance and speed.

Use this guide to get tips on driving safely in fog, and understand when to use fog lights.

What is fog?

Fog is a cloud at ground level that causes a reduction in visibility to less than 1000 metres.

Fog can be thin or thick, when visibility drops to less than 180m. When visibility falls to below 50m it is called dense fog.

What causes fog?

Fog requires both water vapour in the air and pollution particles to form.

Water vapour condenses around these microscopic solid particles to form fog.

The thickest fogs tend to occur in industrial areas where there are more pollution particles in the air allowing water droplets to coalesce and grow.

Sea fog, which shows up near bodies of salty water, is formed as water vapour condenses around bits of salt.

Depending on the humidity and temperature, fog can form very suddenly and then disappear just as quickly. This is called flash fog.

Is fog the same as mist?

Fog is not the same thing as mist. Fog is denser than mist and reduces visibility to 1k or less. Mist can reduce visibility to between 1k and 2k.

Fog Lights

What are fog lights?

Fog lights are extra front and/or rear lights on some vehicles that are designed to make it easier to see and be seen in foggy conditions.

• Front fog lights are usually white or yellow lights which sit below your headlights and can cut through fog without causing glare.
• The rear fog light is a red light, similar in brightness to a brake light, on the back of your car which helps other drivers see you.

Fog lights are designed to shine downwards to illuminate the road below any hovering fog.

The top of the beam is cut off sharply so that the lights won’t reflect back off the fog.

They’re mounted lower down on the car so they can shine underneath the fog.

When to use fog lights?

Fog lights need to be used at the right times to combat reduced visibility, otherwise you could be a danger to yourself and other drivers.

Using them at the right time should aid safe driving but using them at the wrong time could mean you are breaking the law and endangering other drivers.

According to the Highway Code (rule 226) fog lights should only be used in the fog, when visibility drops below 100 metres or 328 feet –  roughly the length of a football pitch.

Never use fog lights when visibility is clear. If you do, you could be given a Fixed Penalty Notice with a £50 fine.

How to turn on fog lights?

Look for a button or switch on your dashboard with the fog light symbol(s) on them, often next to the dial you use to control your regular lights. Some cars control the fog lights through the steering wheel stalk. Look for the fog light symbols and twist the stick to the right position to turn them on.

Front fog light symbol

The front fog light symbol shows a lamp with slanting lines of light in front of it, pointing left. The lines of light are intersected by a wavy line which represents the fog.

Rear fog light symbol

The rear fog light symbol is the same as the front fog light symbol but in reverse. The light and fog lines appear after the bulb, pointing right.

How to drive in fog

Slow down

• Fog reduces visibility, so it is important to slow down to give yourself more time to react to other vehicles and when approaching bends on the road. R
• Reduce speed gently though; don’t slam on the brakes if you hit a thick patch of fog, because that won’t give traffic behind you time to react and they could run into the back of you.
• Be sure to check your speedometer regularly. Fog can create the optical illusion of driving slowly. This causes many drivers to speed up. Don’t rely on your eyesight, but check the speedometer instead.

Use your foglights

• Use fog lights and dipped headlights.
• Use your  fog lights if visibility is reduced to 100 metres (328 feet) or less.
• Do not use full beam, because the fog reflects the light back, reducing visibility even further.

Pay attention

• Driving safely in fog requires paying careful attention to your surroundings. Reduce all distractions. Turn the radio off. Ask your passengers to be quiet and watch for obstacles.
• It’s easy to drift out of your lane while driving in fog. Watch the lines on the road or stay on the right side of the road to ensure that you stay in your lane. Do not change lanes or pass other cars. Be aware of the position of oncoming traffic.
• Open your window a bit to hear outside; this improves awareness of things near your car.
• Be prepared to stop rapidly. Vehicles, people and bikes can appear from nowhere. Some animals also feel bolder in the fog and may enter the roadway.

Keep your distance

• Instead of leaving the usual two-second gap between your car and the one in front, leave at least a four second gap to give yourself more time to react if they suddenly slow down or make a surprise manoeuvre.

Keep your windows clear

• Foggy weather causes condensation and that can build up on the inside and outside of your car’s windows, so use your windscreen wipers and set the car’s heater to demist to keep the inside of the windscreen clear. Keep the heater on a warm setting too, because it’ll help the demist function work better.
• Make sure to use your wipers and to keep your windscreen demisted to aid visibility

Listen for traffic

• When visibility is very limited, it’s a good idea to open windows so you can listen for traffic before pulling out at junctions.
• Hearing other traffic may also help with concentration when driving in fog.

Don’t hang onto another car’s tail lights

• Do not navigate by using the rear lights of the car in front.
• Focussing on the car stops you from being aware of what else is going on around you.
• If the car in front makes a mistake you could well follow them and potentially have an accident.

Delay your journey

• If conditions are severe before you set off delay your journey to when the fog has cleared.
• If you are already en route when the conditions worsen, consider pulling off the road to a safe place such as a service area to wait until the weather improves.

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