Yeah, it's a bit complex. As I recall, the UK story was a bit biased in its presentation - if it's the same one that I'm thinking of. I was actually hoping you *did* have some research to support that (I have my reasons!) and I'd have been all about reading those studies.
Specifically, I think the UK removed it because it cause more hassle than it was worth. There are some litigation issues and they have a bit different justice system there than we have in the US which means I'm not sure of all of the specifics. That might also have been the town that was ripping them out because they weren't paying for themselves well enough - money was going to a third party system provider.
In the US, you're accountable for your vehicle - even if you're not driving it. The only time you're not accountable is when you have not granted authority to use the vehicle. That would be a valid defense to contest an automated speeding ticket but the burden of proof for civil matters is much lower and the 6th Amendment does not grant legal council in civil matters - only in criminal matters. In the UK, it's a little less cut and dried.
Here, in the US, you are responsible for the actions taken by the person you authorized to use your vehicle. It's shared culpability and you could, theoretically at least, then sue the authorized party to recoup your losses. As far as I know, neither of those is *strictly* true in the UK. In the US, for example, some jurisdictions may opt to file criminal charges if you allow an unlicensed person to operate your vehicle. I don't know if that's true in the UK.
If it's the story that I'm thinking of, there was also some backlash and vandalism. People were going out of their way to react to the camera - including speeding just to blow the numbers up and then the owner giving an alibi and saying that it must have been unauthorized use. Which is amusing, truly. I don't think that can be written as having no effect at all.
I still have some ties to the transportation/traffic industry, even though I sold and retired, and being able to throw down some hard numbers would be awesome. Right now, I'm a bit limited to saying that it's not unconstitutional but that unmanned enforcement takes away the accused right to face their accuser. A person who watches a video feed (what they usually use) is not your accuser - they're a reporter. So far, that's gotten me exactly nothing.
It's also important to note that there are cultural differences with traffic behavior. They're much larger differences, or can be, than many people realize. I posted a few comments in this thread about that and, in case you don't know or haven't guessed, I was heavily involved in traffic modeling (thus consulting, engineer, data acquisition, optimizing, predicting, etc) for a long time and I sold back in 2007.
The studies that I have read, and am familiar with, apply to the US. There are some agencies who will do so but I can tell you, it's foolish to try to make assumptions like that. There are very real and very different influences that go into traffic behaviors. So, you may well find a country where drivers had no reaction or even were able to show that it offered no improvement at all. Even a single State, in the US, can have vastly different results. To extrapolate across continents, without doing further research, is a fools errand - so you may find there's an area where it has no effect. I just don't know of any.
At any rate, sorry for the length but that doesn't all fit well o a bumper sticker and I kind of like the chance to discuss (and learn!) what my career was based on. It should be noted that I am of the opinion where I'm unconvinced that the benefits outweigh the fact that this not only strips away the right to face the accuser (not an accuser) and that such an interpretation was upheld by the SCOTUS.
Thanks for the feedback, it gives me food for thought. So far, I've only seen limited area studies. I wonder if they're cherry-picking locations to avoid giving a more complete story. No, no I'd not put that past people to do that. And, oddly enough, Slashdot even has a resident who's fairly familiar with the traffic and transportation industries. Safety, regulation, law enforcement, and much, much more go into modeling traffic. Feel free to respond (obviously) if you want me to fill your screen with mostly useless information about how the industry got to be where it is today and what it entails. You're probably not that bored. I can't say that I blame you.