You should switch your rear fog lights on when visibility drops below

Explanation: If visibility falls below 100 metres (328 feet) in fog, switching on your rear fog lights will help following road users to see you. Don't forget to turn them off once visibility improves: their brightness might be mistaken for brake lights and they could dazzle other drivers.

Category: Vehicle Handling

References: The Official DVSA Guide to Driving: Section 12, Highway Code: Rule 114, Highway Code: Rule 226