While driving a truck, when are you obliged to switch from high-beam to low-beam lights?

In the illustration, you can see the symbol for switching from main beam (high beam) to dipped headlights (low beam). You must switch in this way when another vehicle is approaching from the opposite direction - answer A.

The Road Traffic Act states that a driver using main beam must switch to dipped headlights when a vehicle is approaching from the opposite direction (and if one of the drivers has switched off main beam, the other must do the same), when approaching a vehicle in front if its driver could be dazzled, and when a rail vehicle or a water vessel is travelling so close that you could dazzle their drivers. Rain alone (answer B) is not listed among the reasons for switching the lights, but when rain reduces visibility, a separate rule requires you to drive with dipped headlights or front fog lights - so main beam is out anyway. Entering a built-up area (answer C) is also not a reason in itself: what matters is whether the road is lit. In a lorry the headlights are mounted high, so switch them earlier than drivers of passenger cars do.

Legal basis: Art. 51 para. 3 point 1-3 and Art. 30 para. 1 point 1 letter a of the Road Traffic Act (Prawo o ruchu drogowym)