May the devices forcing slow driving beyond this sign not be marked with warning signs?

In the photo you are driving along a street in town, with a blue sign D-40 "residential area" on the right. Beyond this sign, devices forcing slow driving might not be marked with warning signs. The answer is: yes.

The Regulation on road signs and signals states that sign D-40 "residential area" marks the entry to a residential area and also informs you that devices and measures forcing slow driving placed in the area might not be marked with warning signs. This means speed bumps, narrowings, chicanes or raised junctions - in normal traffic a speed bump is preceded by sign A-11a, but in a residential area it can be left out, because a speed limit of no more than 20 km/h applies there anyway, and at that speed a speed bump is not dangerous. So sign D-40 itself is one warning covering them all: beyond it, you assume that there may be a speed bump around every bend. The Road Traffic Act adds that in a residential area a pedestrian may use the whole width of the road and has right of way over vehicles, parking is allowed only in marked places, and when leaving the area you are joining traffic (włączanie się do ruchu).

Legal basis: § 58 para. 1 and 2 of the Regulation on road signs and signals; Art. 20 para. 2, Art. 11 para. 5, Art. 49 para. 2 point 4 and Art. 17 para. 1 point 1 of the Road Traffic Act (Prawo o ruchu drogowym)