No it isn't - it's a statement of fact, along with the fact that speedometers aren't 100% accurate - yet the eco-nutters will demand zero tolerance of speeding.
I don't see anyone demanding a move from the 10% + 2mph system. Do you have a reputable source stating that anyone is actually campaigning for that? Plus it's not specifically to do with 20mph - indeed, accuracy reduces the faster you go - hence 10% rather than a fixed variation.
It also breeds paranoia in less confident drivers - you already see this on those who stick rigidly to just under the existing limits - so with 20mph they'll be backing traffic up at 15mph because they are afraid / paranoid about getting a ticket for doing 21mph. Now you might argue such people shouldn't be driving - but that's a pretty discriminatory attitude.
We do operate a deliberately discriminatory system for road use - we expect a basic level of skill to be reached before driving alone. There is certainly a bit of a skills issue there, but if there is then making the test harder and introducing other measures to improve skill (not sure what they'd be) is the solution, not saying "we can't have 20 limits because some people will drive at 15mph".
The thing I find most frustrating myself in lower limits is that people seem to just drive in a more leisurely way generally, FWIW. Personally I accelerate the same in any limit, it just tops out at the relevant limit, subject of course to road conditions. Because 20 involves a lower gear, that means I'll be from a stand to 20 in almost no time.
Went on a weekend away to Milton Keynes earlier this year and was great fun cycling into the city from our hotel at the stadium on the Santander bikes - just a shame it seemed many opted for the car and the cycle tracks were relatively quiet.
There are a few reasons cycling hasn't reached its potential in MK, one of them can be solved (it's really quite hilly, e-bikes are the fix), another is a cycle theft problem (secure storage facilities would help here, i.e. Dutch style bewaakte Fietsenstallingen - staffed parks where you have to show a ticket to get out) but there are also the difficult ones to solve i.e. it being very distributed, meaning you need to cycle a long way, which most people won't do.
Some also perceive a crime problem on the Redways, but this doesn't really exist. Criminals don't target them because there aren't enough people on them to be worthwhile!
How about revisiting those places in 20 years time to see whether they were successful or not ?
Too often the high minded idealism hasn't worked out in developments, be it the New Towns of the 1950s, the High Rises of the 1960s, the mixed developments of the 1970s etc - none have stood the test of time particularly well.
There are plenty of nice estates in MK with local centres that have been there more than 20 years. Furzton is one, I started my time in MK there and would happily live there again. There are also plenty of places with established local centre style parades of shops - Maghull is one. Just building a set of local shops, a surgery and a primary school isn't going to turn an estate into a dump.
No - the claim being made by the proponents of 20mph is it reduces casualties - the findings of Queens are that the reduction shown in the immediate years are not statistically significant i.e. well within the usual fluctuation you might see from one year to the next - or in other words, had the speed limit been left unchanged at 30mph you may well have seen that reduction in casualties in any case.
Given that it's quite obviously going to hurt less if hit at 20 than 30, what's the definition of "casualty"?