In my view speed cameras are often set in locations where it is reasonably safe, driving to the conditions, to go faster than the posted limit. Also, quite often, speed limits can be set in way that it is easy to miss a change in limit. The speed cameras are often set quite knowingly at these points.
I agree that speed should be controlled on roads that pass through villages, or points that are dangerous, but often to have a speed limit set to 20 on a major road and then to place a speed camera within yards of the change of limit is nothing but a revenue earner. Speed limits that are justified by the presence of a school that can be enforced even in the middle of the night or during school holidays is another good example.
Every speed camera earns huge amounts of revenue, often amount to millions. Whilst some may be justified many are simply revenue earners justified by a self righteous "well you broke the law so you should pay".