Why must you reduce your speed now?

You need to reduce your speed because there is a truck ahead, which means your lane may slow down or even require you to stop if traffic comes to a halt. In German traffic rules, you must always adjust your speed to the traffic situation ahead, including slower vehicles or traffic jams. The presence of the truck directly in front increases the likelihood of sudden braking or changes in speed, so reducing your speed ensures you can react safely. Since only the truck was the correct reason, neither the speed limit nor the blue car were relevant for the need to slow down in this specific scenario. Always watch for vehicles ahead and be prepared to adapt your speed early to avoid dangerous situations.