Our site roads at work are 20mph limited, and especially for contractors, speeding where is an instant ban from site (which in most cases effectively means they lose their job), nobody breaks the limits. The Civil Nuclear Constabulary do enforce the site limits*
It feels strange driving (and cycling - we also have to stick below 20 officially) at that speed for a while, but now it's just how it is, and you just get used to how the car feels. In many ways, I think the more these 20mph zones spread around the country, the more that people will get used to them and drive to the limits. For now we are still in the 'war on motorists' sulking phase. What's needed is the police to do a bit of enforcement to make drivers realise they do have to take the limits seriously.
*Partly through boredom - they have also been known to chase unlidded cyclists** on blues and twos!
**Site rule annoyingly is mandatory magic hats - a rule which pretty much killed off our fledgling staff 'site bikes' scheme