Use dipped headlights at all times and windscreen wipers and demisters
According to the Highway Code, you must use headlights when visibility is seriously reduced – generally when you cannot see for more than 100 metres (328 feet) or the length of a football pitch.
There's no obligation to use fog lights but if your car is involved in an accident in reduced visibility and its fog lights weren't on, then it may be queried by an insurer.
If you do use fog lights they must be switched off when visibility improves. This applies equally to front and rear fog lights.
When there's fog around visibility can seriously deteriorate in a matter of seconds. Be extra vigilant and drive only as fast as conditions allow and maintain a greater distance between you and the car in front.
Use common sense when it comes to fog lights. Some drivers are worried about dazzling other motorists, and some simply don't know how to turn them on.
Generally it's better to be safe than sorry, so use them when appropriate. Don't keep switching them on and off, though – this can be a distraction, so wait for a consistent improvement in visibility before switching them off - front and back.
Don't rely on automatic lighting in fog during the day.
In fog the ambient light level can be high enough to stop automated lights coming on even though your vision and your vehicle's visibility may be reduced to dangerous levels.
According to the AA-Populus Motoring Panel (October 2015) almost one in ten drivers have automatic lights and rely on them entirely. In other words, when it's foggy every tenth car on the road could be virtually invisible.
(updated 2 November 2015)