Blitzer stood on private property

Hello all, I just happened the following. This morning I flew a little deep and was flashed. On the situation: It is a very wide well developed road with very little traffic. So it is actually no one there who could be endangered. The road is inside, allowed are 50Km/h. The speed camera stood about 300m from the exit sign. Now to the actual question: It was flashed out of the car. From the space conditions it would be This was not the case, however. He was standing on the left side of the road in a private way. Now, for me, the question arises whether this is permissible, that they flash from a private property. Basically, it is the case that authorities have no authority on private property. If necessary, would a permit be required by the property owner? Does anyone have experience with similar facts? Many thanks for Your clues.