British readers of this blog will be well aware of these signs. They measure your speed as you approach them and remind you of the speed limit if you are exceeding it. They don’t send a ticket if you are speeding, they are just a reminder. They are moved to different locations where speeding is prevalent.
The local authorities or Highways agency would do well to occasionally move them to locations where slow driving is prevalent. Drivers dawdling at 25mph on a road where the speed limit is 40mph are a hazard because they annoy other drivers, some of whom will take undue risks to overtake.