Fog warning signal: meaning and rules

What does the fog warning signal mean?

Know your traffic signs lists this signal as “risk of fog ahead”. Rule 234 says if Fog is shown on a sign but the road is clear, be prepared for a bank of fog or patchy fog ahead.

Quick quiz

What does the sign at the top of this page mean?

An advised speed for the conditions, not a limit.
The temporary restrictions end here.
Risk of fog ahead.

Show answer

C. Risk of fog ahead. The signal warns of a risk of fog ahead (Know your traffic signs). Source: Know your traffic signs: motorway signs, signals and road markings, GOV.UK; The Highway Code, rule 234, GOV.UK; The Highway Code, rule 226, GOV.UK; The Highway Code, rule 236, GOV.UK; Traffic sign images, Department for Transport, GOV.UK

Where will you see it?

On motorway signals and variable message signs.

What should you do?

Check your mirrors and slow down. Keep a safe gap and be ready to use your lights.

What do people get wrong?

Keeping your speed because it is clear where you are. Fog can be thick just ahead.

Common questions

Should I put my fog lights on?

Only when visibility is seriously reduced (rule 236), which rule 226 describes as generally when you cannot see for more than 100 metres. Switch them off when visibility improves.

What does the Highway Code say about fog on signs?

Rule 234: be prepared for a bank of fog or drifting patchy fog ahead, even if the road is clear.

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Sources

Checked on 4 November 2026 against Know your traffic signs (motorway signs, signals and road markings) and The Highway Code rules 234, 226 and 236.

Contains public sector information licensed under the Open Government Licence v3.0. Traffic sign images are Crown copyright, from the Department for Transport. Sources and image credits.