At 20mph lower than the standard speed for that type of road will not reduce the speed by much - I agree. Research has previous pointed to the fact that people only alter their speed by 1mph...
If the problem is the behaviour of drivers/road users then they are not addressing the real problem but trying to fix something that doesn't need fixing.
It's all too easy to 'blame speed', when it is the behaviour of people that is really in question.
If someone's judgement is no longer 'good' then that needs addressing.
VAS /SID are good for roads and help teach by reminder drivers of their speed.
But speed isn't the main problem it's behaviour to the environmental conditions.
And I'd hazard a guess that it's the occasional driver not the vast majority.
So again - if they spent the money on a few good police patrols then this would likely see a reduction in 'bad behaviours'.