In the UK we have (only a recent thing, so afaik it's not massively common yet) "average speed" cameras - they take a photo of you at two points along a road, a

In the UK we have (only a recent thing, so afaik it's not massively common yet) "average speed" cameras - they take a photo of you at two points along a road, and calculate your average speed by how long it took you to get from A to B, then ticket you for speeding if it was too quick. That prevents people from just slowing down when there's a camera coming up.