Does this sign permit a vehicle that runs on natural gas to enter the green transport zone?

In the picture there is sign D-54 "green transport zone" (strefa czystego transportu): a white rectangle with the word "Strefa" (zone) and a green circle with a car symbol. A vehicle powered by natural gas may enter a zone marked with this sign. The answer is: yes.

The Regulation on road signs and signals states that sign D-54 "green transport zone" marks the entry to a green transport zone. The rules of entry are set by the Act on Electromobility and Alternative Fuels: electric vehicles, hydrogen-powered vehicles and natural gas vehicles may enter the zone, as well as vehicles exempt from the restrictions, such as Police or rescue service vehicles; the municipal council may also allow other vehicles in its resolution, for example combustion-engine vehicles that meet a specific emission standard. Natural gas (CNG or LNG) is listed directly in the Act as a fuel that gives the right to enter, so such a car may enter every zone in Poland, regardless of the content of the resolution; the municipality can only require a sticker on the windscreen or check the type of fuel in the vehicle register. Note the difference: LPG, that is the popular autogas system, is liquefied petroleum gas, not natural gas - a car with LPG is treated as a combustion-engine vehicle, and whether it may enter is decided by the municipality's resolution.

Legal basis: § 60d of the Regulation on road signs and signals; Art. 39 of the Act on Electromobility and Alternative Fuels