Is it permissible to drive at a speed that could make it difficult to stop the vehicle when the red signal starts flashing?

Why?

You must approach a level crossing at a speed that lets you stop in a safe place if a signal prohibits entry. The frames show raised barriers and crossing signals, but you must still be ready to stop. A speed that makes stopping difficult when the red light comes on is not permitted.

art. 28 ust. 2 Prawa o ruchu drogowym