What's the maximum speed at which you can ride a motorcycle once you go past these signs on the road?

In the photo you are riding on a dual carriageway whose roadways are separated by a concrete barrier; on the right there is sign D-7 "expressway". On a motorcycle you may ride on this road at a maximum of 120 km/h - answer C.

The Road Traffic Act states that the speed limit for a motorcycle outside a built-up area on a dual-carriageway expressway is 120 km/h. The Regulation on road signs and signals states that sign D-7 means the beginning or continuation of an expressway. The answer 100 km/h would apply to a single-carriageway expressway, and 90 km/h to an ordinary road outside a built-up area - but here you can see two roadways separated by a barrier, so the 120 km/h limit applies. On an expressway and a motorway you must also keep a following distance from the vehicle in front of at least half your speed in metres: at 120 km/h this is 60 m. At such a speed a motorcycle is very sensitive to gusts of side wind and to ruts, so keep both hands on the handlebars and avoid riding right behind large lorries.

Legal basis: Art. 20 para. 3 point 1 letter b and Art. 19 para. 3a of the Road Traffic Act (Prawo o ruchu drogowym); § 48 para. 1 of the Regulation on road signs and signals