I'm not sure whether I have followed your example correctly, but interpret it as being that the local population was essentially law abiding, because they knew

I'm not sure whether I have followed your example correctly, but interpret it as being that the local population was essentially law abiding, because they knew the camera locations both when they were visible and when they were non-visible, and that the main issue was with out-of-town drivers. There is, of course, no reason why a local council should not decide to deploy visible cameras in such a situation, alongside a national network of non-visible cameras (you obviously don't need to have both visible and non-visible cameras at the same site). The cost-effectiveness argument still holds, under the assumptions previously mentioned.