Generally only on the approach to the crest of a hill, or a bend.Few things, there is no property in a witness or evidence, so therefore it is not used against you, but more to establish a fact. So pointing a camera at a speedometer and then speeding is providing evidence that an offence is being committed. Crossing a white line is again an offence, and one that should not be done, it's such an easy 3 point offence, it's not worth doing. They are usually there for a very good reason, so please don't cross them.