Straw man argument
The automatic corrollary to saying that the figure published is not the right figure is not that the "right" figure is the pubished figure less the number just owned up to.
The actual state of play is that the DfT had evidence that said: at speed cam sites, the reduction in KSI after putting a speed cam in place is x%. The amount of reduction definitely NOT attributable to the speed cam is y%. Therefore the amount of reduction that MAY be attributable to the speed cam is (x-y)%.
Not IS, but MAY. However, i am also aware of work that challenges any claim that the true reduction in KSI is (x-y)%. So it is fair to say that the oft-quoted figure is wrong: not fair to say that we now know that the real figure is whatever is left.
As someone with a healthy respect for stats, this bothers me, because it feels very much like the DfT has cut the ground from under its feet. If they had come clean in the first place, then people would be far more likely now to buy into the reduced figure. As it is, they are in the position of saying: sure, we knew our figures were misleading, but we published them anyway...
Likelihood of people having great confidence in what is out there now? Not great would be my guess.
The irony is that places like Oxfordshire DO appear to have done some of the regression to the mean studies needed...whereas some other partnerships (Devon and Cornwall spring to mind) haven't. Yet, on the basis of what is quoted by officials, it is next to impossible now to separate out junk stats from decent peer-reviewed and critiqued invetigations.
That is a breeding ground for junk stats from all sides, making a total nonsense of the debate. But it could have been all different: if the DfT in the first place had taken note of the various critiques made of their work, done a longer-term study using independent academics considering each of the methodological criticisms made - and maybe also published a gold standard methodology for assessing the utility of speed cams...well, not everyone would have bought into that, but a lot more would.
So if you ask me what the real story is here? A well-intentioned government department getting its comeuppance for being too superior, too economic with the truth - even if that was done at some point for the best possible motives.