How oten it happen, I don't know, but there are big signs on the highway reminding you every now and then. There are also signs that say "aircraft patrolled" in lots of places, but not many speed cameras - mostly just red light cameras.
Some bloke got caught here the other day doing 219km/h (136mph) in a 90km/h (55mph) zone. His car got impounded for a week, but apparently he's kept his licence until the court case in September. Mind you, he's facing his car being scrapped and dangerous driving charges when it does get to court.
if there are 100s of km with no-one else around why does it matter if someone is speeding?
and, by cost effective, I was wondering about the cost of maintaining the aircraft, pilot etc against the revenue it generates
Because it's against the law, because there are other drivers on the road who might be endangered, because in some areas you could end up smashing into something like a moose (which can kill you even at much lower speeds), in the winter, road conditions aren't optimal, etc.
I can see how the "points" part of the original title could be turned into a full stop, but how the "speeding" part ended up in a Sanatorium sentence, I don't know.