That's exactly why they are visible. People don't slow down because they abstractly understand that speed cameras exist in the universe. They slow down where they know there is a camera, and that is exactly the behaviour that is being exploited. Cameras are intended as a location-specific safety intervention that (according to all practical data) makes drivers slaughter vulnerable road users significantly less than when there isn't a camera. You can argue that it's a bandaid but I think that would mean you were trying to solve a problem other than the one the camera was deployed for.