A bright rear fog light can help another driver locate a car in seriously reduced visibility. In clearer conditions, that same light can be dazzling. The useful skill is knowing both when to turn it on and when to turn it off.
Learn the controls before the journey
With the car safely parked, identify the front and rear fog-light controls in the owner’s manual. Not every vehicle has both. Check which dashboard indicator corresponds to each function and whether another lighting setting must be selected first.
Do not assume that automatic headlights also make every fog-light decision. A lighting system’s behaviour depends on the vehicle, and daylight fog can still create poor visibility. The manual explains what the system does; the driver must assess the conditions.
Use the rule for the road you are on
As one jurisdiction-specific example, the UK Highway Code describes seriously reduced visibility as generally less than 100 metres and requires fog lights to be switched off when visibility improves. Other countries have their own requirements. Check local rules rather than exporting a familiar threshold everywhere.
Fog lights do not create extra stopping distance. Reduce speed appropriately, leave room and drive so you can stop within the clear distance ahead. Do not follow another vehicle’s rear lights as if they guarantee a safe speed or route.
Review the setting when the conditions change
An invented journey might begin in dense fog, cross a clear ridge and then return to a misty valley. The setting that suited the first section needs reassessment on the ridge. A dashboard reminder is useful only if you notice it.
When you arrive, check that the lights are no longer selected. If a lamp or indicator behaves unexpectedly, consult the manual or a workshop rather than improvising wiring changes.
The aim is to make the vehicle visible without creating a new problem for someone behind you. Familiar controls, a clear view and repeated attention to changing conditions matter more than leaving every available lamp illuminated.