At what distance can speed cameras catch you?

[XRchic]

I was wondering how close you have to be to a speed camera before if can nab you. Is it 50m or so or even up to a km away??? Obviously line of sight is required, but distance I am not sure about.

Also, can they get you in both directions at once?

Reason I ask is because I was coming down the freeway and spotted the camera about 500m away, down the hill. It was facing me, on the other side of the road. I slowed down pretty quick but I dont know if a) I was too far away and b) whether it was only going for the uptrack cars... or both.

I guess I will find out in a week or so!

Thanks