Returning to the Sam Riddall/Bristol case. A 20mph zone MAY have saved his life. The point that the driver was ignoring the speed limit is not that relevant. 20mph zones as they are enforced in London do not rely on driver's compliance with a speed limit sign. They rely on passive calming methods - humps mostly. The study shows they work. Average speed is reduce 9mph to 17mph.
I presume 'twice the speed limit' means 60mph in a 30mph area. I doubt whether the lady however mad would have attempted anything like that in such a zone. So we go from certain death @60mph to at least a chance if those calming measures kept to around 30 or less.
We have humps in my town as well. They are spaced so the largest vehicles (buses) can pass over them without having to lift off the gas at all (good idea that 
) I regularly see cars travelling at 50+mph in 30 zones - usually driven by kids (or nicked)
It would be nice if people voluntarily agreed to honour speed limit signs. It is sad for those that do that they have to suffer the discomfort, wear & tear on vehicles and ironically unneccessary deaths because emergency vehicles cannot get to a life saving situation (like a car crash) fast enough. But the study gives very strong evidence that it saves even more lives.
You said it, if they are going ignore a 30, then there is every likelihood they are going to ignore a 20. If peds are off their guard in a 20, then this puts them at even great danger from those who will chose to ignore them. It cuts both ways.
I don't see what arguements there can be against the expansion of 20mph zoning. It will be limited by the cost of calming and we probably need more studies to highlight which areas would benefit most and which calming methods work best. Lets try and push for commitment to get those measures written into the election manifestos of the two major parties.
Speed cushions are a danger to motorcyclists. You won't feel them on 23mm wide cycle tyres, but I can assure you that on a 120mm wide front tyre on a motorcyle, they do present a substantial risk from deflection if you hit them at anything other than directly square, which is very undesirable. I would fight against such blanket measures and think your stance is a fairly self serving one.
Maybe just for the pleasure as they squirm knowing that it is electorally difficult to alienate motorists. But the evidence is clear ... and all this global warming stuff is predicated on statistics that politicians say is convincing when this is even more convincing!
