You are driving with your front fog lights switched on. Earlier fog has now cleared. What should you do?

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When to use fog lights
Some drivers appear to believe that the slightest mist is significant enough to switch on fog lights, but this is not the case. So let's go through when you should use them and, more importantly, when you shouldn't.

Fog lights are intended for use when vision is severely limited, most notably by fog, but also in other poor visibility circumstances such as snow or heavy rain. They are intended to make you more visible rather than to light your road, so they are not required under ideal visibility circumstances at night - your headlights are for that.

They should only be used when your vehicle's main lights are insufficient to make you visible to other drivers. According to the Highway Code (Rule 226):

"You MUST use headlights when visibility is seriously reduced, generally when you cannot see for more than 100 metres (328 feet). You may also use front or rear fog lights but you MUST switch them off when visibility improves."

Street lights on 30mph roads in the United Kingdom are placed no more than 200 yards (183 metres) apart. If you can see the next street light up the road from the one you're passing, that's a good rule of thumb for utilising your fog lights. Using this rule, it's evident that fog lights should only be employed in the most foggy or snowy situations.

Of course, fog lights are a useful component when visibility is low, but they will not make you invincible. Use common sense: if you can't see very far ahead, slow down and maintain a safe distance from the car in front of you. If there is fog, the roads are likely to be slick, so leave more room to brake and slow down.

When NOT to use fog lights
If the fog has dissipated, you must turn off your fog lights to avoid blinding other drivers. Keep an eye on the weather - while fog normally occurs in the early morning, it will disappear as the sun rises, so if you started your day with your fog lights on, make sure you continue to assess a situation and switch them off when the weather clears.

If you've broken up your drive, the fog lights may have shut themselves off when you turned off your car's ignition, and they will only come back on if you manually turn them on. However, if you must drive with your headlights on, make sure they are completely turned off.

Rule 236 of the Highway Code states explicitly when fog lights should not be used:

"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."

The key idea here is to avoid concealing brake lights. Because rear fog lights are brighter than regular tail lights, other drivers may not see your brake lights illuminate when you brake.

Fog lights are obviously unnecessary when it's dry, and they're also unnecessary when it's dark. Because the lamp housings have the same reflectors as main beam headlights, they are unlikely to provide extra light to assist you in seeing where you are going.

Another no-no is utilising fog lights in damp weather. The increased brightness of fog lights is multiplied by their reflection off a wet road surface, whereas the bright red light can produce glare through other vehicles' windscreens when it rains. Front fog lights can potentially be a source of distraction, particularly if they are poorly adjusted and produce dazzling reflections.