What must you do on encountering sudden dense fog driving at daytime?

When encountering sudden dense fog during the day, you must adapt your speed immediately to the reduced visibility to maintain safety and reaction time. Additionally, switching on your dipped headlights is essential because it increases your visibility to other drivers without causing glare. Using only parking lights is insufficient and not allowed, as they do not adequately illuminate the road or make your vehicle visible enough in foggy conditions. Following these rules helps prevent accidents in sudden fog.