At what maximum speed are you allowed to drive on the road beyond this sign if you are hauling another passenger car?

Why?

The white D-42 sign with a silhouette of buildings marks entry into a built-up area. When towing another car, the speed limit there is 30 km/h. Driving at 50 km/h would exceed the towing limit, while 60 km/h is permitted when towing outside built-up areas.

art. 31 ust. 1 pkt 1 Prawa o ruchu drogowym; znak D-42 „obszar zabudowany”