Who does the speed limit expressed by these signs apply to?

In the picture you can see sign B-33 "speed limit" of 50 km/h with plate T-23a showing a motorcycle. This limit applies to a motorcycle rider - answer B.

The Regulation on road signs and signals states that sign B-33 means that you must not exceed the speed shown on the sign, and the text or symbol placed on a plate under the sign is an integral part of the sign; plate T-23a shows motorcycles, so the ban applies only to motorcycle riders. The plates in the T-23 series narrow down who the sign is for: T-23a is a motorcycle, T-23b a lorry, T-23c an agricultural tractor and so on. A moped rider (answer A) is not a motorcycle rider - a moped is a separate type of vehicle, with an engine of up to 50 cm3 and a speed limited to 45 km/h - so this sign does not apply to them, although they will not go faster than 45 km/h anyway. Answer C reverses the meaning of the plate: a sign with T-23a applies only to motorcycles, and all other power-driven vehicles drive here according to the general limits. Such signs are found on roads with narrow, slippery curves, where two-wheelers are particularly at risk.

Legal basis: § 27 para. 1 and 2, § 23 para. 2 point 1 and § 2 para. 4 of the Regulation on road signs and signals; Art. 2 point 45 and 46 of the Road Traffic Act (Prawo o ruchu drogowym)