But once people know they are there, they will just brake before the additional sensors, and speed up after them. moving the 'problem' earlier or later,.
Ok, so it extends the monitored area, in which case they may as well have just put two more cameras or an average speed camera there instead,
and if we are to believe that "they are only placed in an accident blackspot" then that single camera is achieving its objective, because excess speed before and after it, has historically never been an issue.