Is it mandatory to maintain a minimum distance of at least 100 metres from the vehicle in front on a road marked in this manner?

Why?

Sign D-9 indicates a motorway and does not impose a fixed minimum gap of 100 m. Except while overtaking, the required gap in metres is at least half your own speed in km/h, for example 70 m at 140 km/h. Road conditions may require a greater distance.

art. 19 ust. 3a Prawa o ruchu drogowym