Are you obliged to secure the vehicle parked on a hill against rolling down?

You are obliged to secure a vehicle parked on a hill against rolling away. The answer is: yes.

The Road Traffic Act requires a driver, when parking, to secure the vehicle against being started by an unauthorised person and to take other precautions necessary to avoid an accident; a car rolling away on its own can kill a pedestrian or smash into other vehicles. On a hill you pull the parking brake on fully, in a car with a manual gearbox you engage a gear - first when you are facing uphill, reverse when you are facing downhill - and in an automatic you select position P. You turn the wheels towards the kerb when parking facing downhill and away from the kerb when parking facing uphill, so that if the brake fails, the car rests against the kerb. On a steep slope you also put a wheel chock under a wheel.

Legal basis: Art. 46 para. 5 of the Road Traffic Act (Prawo o ruchu drogowym)