Many drivers are not aware of the law on the use of fog lights and will often misuse them. It is only when visibility is seriously reduced below 100m that they should be used. If you pace that you out you will find it is actually not a great distance at all. The use of fog lights in conditions other than this is against the law, and you can be prosecuted.
In misty conditions, dipped headlights are generally sufficient to make sure that others can see you clearly. Fog lights are very bright, and will simply dazzle other drivers if used incorrectly. As a basic rule, use your mirrors; if you can clearly see the driver behind then they can also see you and your fog lights are not needed.
If the weather is so bad that you do need to use them, you should use them with a bit of thought. For instance, when visibility is really poor you may need to turn them on while driving along in steady traffic. However, as traffic slows down or stops, and the following driver is then closer to you, do you you still need to keep them on? Once another driver is close enough to see you without any problem, switching them off temporarily will prevent unnecessary dazzle. You can switch them back on again once you get moving at a higher speed.
When you have just stopped in particularly bad conditions, as an extra precaution you might consider keeping your foot on the brake. Your brake lights will provide an extra warning to anyone coming up behind. Do take your foot off the pedal however after another driver approached in order to avoid dazzling them!
For more information about driving in fog, read Don’s book ‘Driving at Night & in Bad Weather.’