Is the driver obliged to apply the so-called "zipper merge" when there is no significant speed reduction?

In the photo, a street in a town; you are driving in the right lane behind a silver car and a lorry, and your lane is about to end - an area closed to traffic begins on the right - while a white lorry is driving in the left lane, which continues; as the question assumes, traffic is flowing smoothly. The driver is not obliged to use the zipper merge (jazda na suwak) when there is no significant reduction in speed. The answer is: no.

The Road Traffic Act introduces the zipper merge only "in conditions of a significant reduction in speed" on a roadway with more than one lane in the same direction, when driving in a lane is not possible because of an obstacle or because the lane ends - only then must the driver in the adjacent lane let in one vehicle immediately before the point where the lane ends. When traffic is flowing smoothly, the general rule applies: a driver who is changing lanes gives way to a vehicle travelling in the lane they intend to enter. The driver in the ending lane must therefore find a gap on their own and merge without forcing anyone to brake, and the drivers in the adjacent lane are not obliged to let them in.

Legal basis: Art. 22 para. 4 and 4a of the Road Traffic Act (Prawo o ruchu drogowym)