In the visible situation, is it necessary to turn on the direction indicators to signal change of traffic lane?

In the photo you are driving in the right lane of a multi-lane one-way roadway alongside noise barriers; on the right there are signs B-36 "no stopping" and D-3 "one-way road", and other cars are driving ahead of you. A lane change must be signalled with the indicator - switching it on is necessary. The answer is: yes.

The Road Traffic Act requires a driver to signal an intention to change direction or lane in good time and clearly, and to stop signalling immediately after completing the manoeuvre. The obligation does not depend on whether you can see other vehicles nearby - you always switch on the indicator, because someone may be in your blind spot, and on a multi-lane one-way roadway someone may be overtaking you on the left at that very moment. A signal given "in good time" means a few seconds before the manoeuvre, not during it; only after switching on the indicator do you check the mirrors and the blind spot, make sure that you will not cut in front of anyone, and change lanes smoothly.

Legal basis: Art. 22 para. 4 and 5 of the Road Traffic Act (Prawo o ruchu drogowym)