They're cost effective when you set an arbitrarily low speed limit on a historically fast stretch of road. In order to support the $5000+/mo cost of each speed camera, you need to be handing put speeding tickets lile they're going out of style, which incentivizes doing the first part.

It's the speed trap town, automated. Also an American problem. Build a town that uses the highway as a main street so you can enforce an arbitrarily low speed limit in city limits and catch people that miss the first speed limit change sign. Because 99 percent of traffic is passing thru, they are less likely to fight it in court. Easy money for the people running the town.

Also they used to use all sorts of tricks, like posting multiple speed limit change signs in short sequence (65->55->45->35 in the same 1/4 mile (400m for you metric people)), then making sure at least one of those signs is partially obscured by an overgrown tree or in the middle of a bend or camo'ed between city info signs, where you're less likely to see it while concentrating on everything else.