When driving behind a vehicle that is travelling at a speed of 80 km/h on a road marked in this manner, is it mandatory to keep a minimum distance of at least 40 meters from it?

Why?

Sign D-9 indicates a motorway. The gap to the vehicle ahead in the same lane must be at least half your own speed in km/h, expressed in metres. If you are also travelling at 80 km/h, the minimum gap is therefore 40 m. This calculation does not apply while overtaking.

art. 19 ust. 3a Prawa o ruchu drogowym