If “20mph is extremely popular: 70%+ support 20mph speed limits.” then why is compliance with 20mph speed limits so low?
Originally we were told that 20mph limits would be self enforcing as everyone wanted them, yet now there are many articles on this very site talking about the need for enforcement of 20mph limits / calling for vehicles to be fitted with automatic speed limiting devices.

Dont get me wrong – in the right places and at relevant times I totally support 20mph zones / limits, but in my opinion they have been vastly overdeployed with a corresponding loss of public support.

Firstly “Speed is a feature of every crash where the driver failed to stop in time”
Clearly this is at best an exageration, if the driver isnt looking where they are going or somebody steps out directly in front of a moving vehicle then there is going to be an impact, unless what you are saying is that cars should never move.
Yes obviously increasing speed makes the impact worse, saying speed is a feature in all crashes is like saying the ground is a feature in all fall from height incidents.

Secondly If as this site keeps claiming everybody wants 20mph limits including drivers, then why are 20mph limits so widely flouted?
Surely they should be self enforcing by mutual consent if they are so popular.
Yet according to an article by road safety GB ( http://www.roadsafetygb.org.uk/news/5160.html ) 84% of drivers exceed 20mph limits.

There are many situations where 20mph limits make perfect sense. In Leeds where I live there are a lot of 20Mph zones. There are some 30mph roads where I would never consider driving anyhting like 30mph because in my opinion that is not a safe speed for the road. Most motorists are snesible and considerate, yes there are some idiots, but they will always be idiots irrespective of the speed limit.
In my opinion 20Mph limits have been vastly over used, how does it make sense to have a 20mph limit outside a school at 10pm during the summer holidays? Equally there are lots of perfectly long straight roads with really good visibility wide pavements etc where I simply do not see any justification for a 20 mph limit. Yes I get that cars are not the only road users, I get that children sometimes do stupid things – that’s why you ease off when you see them, I get that older people need time to cross the road safely, thats why you slow down and if necessary stop – in the hope that others will do the same for your kids and the same for you when you are older.
The excessive use of 20mph zones has resulted in the concept being brought into disrepute, people simply ignore them.

In my opinion time related advisory speed limts are a good idea, people reconise them as a genuine high hazard situation and respond accordingly rather than just ignoring a 20mph speed limit because its the hundredth one they have gone through that day and they suspect there is no good reason for it anyway.