What should you do?

Your choice to stay in the middle lane and slow down is correct because, according to German traffic rules, when you approach a lane that is ending (as indicated by advance warning signs), vehicles in the continuing lanes have the right of way. Slowing down while staying in your lane helps ensure a smooth merge for drivers coming from the lane that ends, preventing abrupt cuts or dangerous lane changes. Moving into the ending lane or trying to overtake in this situation is not advised because it can cause conflicts and disrupt traffic flow. Therefore, the best practice is to maintain your lane in the middle (continuing) lane and adjust your speed to facilitate safe merging. This promotes orderly and safe driving on the Autobahn during lane reductions.