What is the right behaviour now?

In this situation, you are approaching a lane closure, which means you must prepare to merge into the remaining open lane. According to German traffic rules, you should reduce your speed when a new lower speed limit is posted and use the full length of your lane up to the merge point. This is known as the zipper merge (“Reißverschlussverfahren”).

Letting the blue car in front of you move over is important because merging vehicles need to be integrated smoothly into the remaining traffic. Drivers in the continuing lane should allow space for cars coming from the ending lane to merge one after another, just like the teeth of a zipper. Accelerating would be wrong here, as it can make merging more dangerous or cause congestion.

Following these two steps—reducing speed and letting the merging vehicle move over—ensures safety and efficient traffic flow when a lane ends.