Hmmm - what about the politics of it all? Would any politician be proposing that - I would think not. That's before you get onto banning cars for residents (who live in the city centre) and the issues of blue badge holders etc.
That's before you then start factoring in the impact on local businesses (why go shopping in Worcester - come drive to Gloucester) or will office firms look to relocate.
There's reasons why such measures aren't being pursued.
Talking of bans/driving restrictions: --
Of the various CAZs proposed around the UK, some seem to be backtracking, I believe (correct me if I have misunderstood) that both Derby & Nottingham have recently decided not to go ahead with a full low emission zone, instead, they are happy to just see buses upgraded to Euro 6.
Local politicians are terrified of P****g off private car & van owners.
Politically, it is relatively easy to persuade the 'Big Boys' to upgrade their vehicles (and I am including Big delivery vehicles/companies here as well as buses) not so small vans, cars, motorcycles etc.
London is looking interesting (By the way I personally fully support the ULEZ and revised outer zones), lots of rumours going around now about a backlash from London residents, we will have to see.
People will also have been watching what has been going on in France.
Like it or not, brutal reality is that many people in the UK believe that they have a right to own/drive as many vehicle as they like and drive where and when they like, this is the dilemma for politicians.