Under HEAVY FOG conditions;- REDUCE SPEED; so you can stop within the distance you can see ahead. Use that speedometer as fog dulls human senses. LOW-BEAM headlights - ON. Use FRONT FOG LIGHTS, if equipped. In severe fog-like conditions; drive with just the park (sidelights) on, AND front fog lights. This action reduces glare and allows the front indicators to stand out more clearly to lead traffic, again check that speed. (Action does not apply to North America, owing different wiring regulations). Switch your cars REAR FOG LIGHT (red) ON. Keep the hazard-warning lights for unexpected crash and breakdown scenes,- per their designed intention. If you frequent heavy fog conditions, you should consider fitting a rear fog light, as the lamp offers up to 20 times the luminous intensity over a vehicles regular taillights. They can be mounted in pairs, spaced equally apart, or as a single unit placed on the driver side edge - in towards the centre of the vehicle. They must NOT be fixed within 100mm from any brake light. Earth the lamp. Run it's second wire to a 3-pole yellow/orange illuminating switch 'ACC' position, this "pilot light" is legally required, and it tells the driver that the rear fog lamp is on! Earth the switch to its Earth pole. Take the 'feed' from your vehicles park-sidelights. Switch the rear fog lamp OFF - the moment you can see well enough ahead. Do not use the rear fog in a light mist, rather keep it for dense fog, dust storms, daylight torrential rain on a highway, in bushfire smoke conditions and the like. Global design rule for the function is UNECE 38, but many nations,- mistakenly in road safety terms, keep the function optional!! {National governments are best advised to mandate the function, giving manufacturers a 3-year notification for compliance}. You can buy dedicated rear fog lamp switches that bear the functions unique ISO symbol, OR buy a generic orange/yellow LED switch. Cheers.