234
Before entering fog check your mirrors then slow down. If ‘Fog’ is shown on a sign but the road is clear, be prepared for a bank of fog or drifting patchy fog ahead. Even if it seems to be clearing, you can suddenly find yourself in thick fog.
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235
When driving in fog you should

• use your lights as required (see Rule 226)
• keep a safe distance behind the vehicle in front. Rear lights can give a false sense of security
• be able to pull up well within the distance you can see clearly. This is particularly important on motorways and dual carriageways, as vehicles are travelling faster
• use your windscreen wipers and demisters
• beware of other drivers not using headlights
• not accelerate to get away from a vehicle which is too close behind you
• check your mirrors before you slow down. Then use your brakes so that your brake lights warn drivers behind you that you are slowing down
• stop in the correct position at a junction with limited visibility and listen for traffic. When you are sure it is safe to emerge, do so positively and do not hesitate in a position that puts you directly in the path of approaching vehicles.

236
You MUST NOT use front or rear fog lights unless visibility is seriously reduced (see Rule 226) as they dazzle other road users and can obscure your brake lights. You MUST switch them off when visibility improves.
Law RVLR regs 25 & 27