It's the exact opposite of what you just said. A brightly painted camera enforces the law - it makes literally everyone follow the rules within its sight. A hidden camera does not enforce the law - it just guarantees punishment, but not compliance. It might make people slow down, but only after they get a ticket - and I'd argue that's far too late. If you have a section of the road where it's absolutely critical to follow the speed limit(near a school for example) then a visible speed camera is a far far far better choice than a hidden one.