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They're the smaller yellow cameras over the lanes or off to the side of the road on a yellow gantry, multiple cameras as it is one per lane. With smart motorways they're generally adding them to the back of the overhead gantries, so you don't even see them.
It's all done with the help of ANPR. So they know when your car passed one set of cameras and when they passed another set and how far they are apart.
This link has photos: https://www.carbuyer.co.uk/tips-and-advice/160228/average-speed-cameras-how-do-they-work
Greenbank
Are average speed checks on motorways bogus? I haven’t been able to see anything set up to measure entry and exit.