Why would you put the speed cameras on highways then? Here they are on 50-60km/h roads, some times 70. On faster roads they are only near crossings where the speed limit is reduced.
Netherlands has speedcameras and average speed checks on a lot of highways. Mostly because it works as a good public statement to claim you have "caught a lot of speeding to make traffic safer" while you don't actually have to hire any extra police officers for it.
That's literally all they do in Israel. They spent hundreds of millions of dollars setting up speed cameras on Highways ONLY, and the only effect is increased revenue.