"So are you one of those people who think that you only need to slow down once you've passed the sign?"
Given that I'm an IAM member, the answer to that is no.
However the point is that the camera van *could* be located in a position where it would be visible *before* you reach the 30mph sign thus ensuring that people slow down *as* they reach it. But, of course, that wouldn't be so good for revenue, would it?
And when you say "As for the obscured camera all you need is photographic evidence and the conviction won't stand", you rather miss the point that you won't necessarily even *know* if you've been caught and get the NIP a couple of weeks later, so if you go back subsequently and take a picture, they'll just argue that "well we say it wasn't in that position when we caught you, let's see you prove otherwise".
As for "your local speed camera partnership advertises when those cameras are in use on their web site", I quote from the Hampshire "Safety Camera Partnership" site:
"Enforcement can take place on any road at any time. Camera locations and routes are approximate.
"Please note, we can carry out speed enforcement on any road at any time."
So, no, they *don't* advertise when they are in use and their map simply shows a length of road (which can be several miles long) where a camera *might* be.
I have no objection to cameras where they will actually *do* some good. I do object when they are placed with the seeming intent of simply raising revenue.