The proposals to restrain general traffic passing through the Centre will inevitably have knock-on effects for other parts of the road network and it will be the capacity of the wider network to absorb displaced traffic that will limit what can be achieved in the Centre. That is not to say that all the displaced traffic will have to be accommodated elsewhere. Some displaced car trips will be made by other modes instead and some will not be made at all, but a proportion of displaced trips, particularly those made by vans and lorries, will still need to be accommodated somewhere.
View City Centre changes - Dec 2009 in a larger map
The map above attempts to identify the alternative routes that will bear the brunt of the displaced traffic. As well as taking out the option of linking from Baldwin Street to the Centre the map shows Prince Street Bridge as closed to motor traffic as is already planned to accommodate the Ashton Vale BRT. The net effect is that journeys by car, van or lorry between the Clifton area and the south of the city become very constrained compared to what is currently possible. There will be almost no practical means of passing between those areas between the Cumberland Basin to the west and Newfoundland Circus (Cabot Circus) to the east.
In the south to north-west direction it appears that a couple of other routes will remain open (shown light blue on the map) along sections of one-way street, via Union Street in Broadmead and via Small Street and Nelson Street (although this latter is very circuitous). Union Street is frequently congested anyway and is an important bus route so it's unlikely that extra traffic can be accommodated there. In the other southbound direction one or two routes are possible but so circuitous as to be impractical, except perhaps the Bridewell St - Pithay route shown yellow which I suspect will need to be closed off to prevent it developing into a major rat run.
Elsewhere we can see that Park Street will become less attractive as a through route since it will lead only to St Jame's Barton, which can be reached more directly via Park Row - Marlborough Street. Indeed this might pave the way towards closing Park Street itself to motor traffic and diverting traffic via Jacob's Wells Road, but that's another story. Clearly Park Row - Marlborough Street will take displaced traffic and given how congested that corridor already is that must present problems.
From St James Barton round to Newfoundland Circus, Old Market (above) and Temple Circus will also take displaced traffic. In addition this is the planned route of the BRT going anti-clockwise around the city centre so there are more capacity issues there. Something has to give if these changes are to be pushed through. That something should not be the economic vitality of the city and we have to recognise that motor traffic, particularly commercial van and lorry traffic, has an important role there.
So that just leaves the private car, the elephant in the room that we all know is the root of the problem but few will dare criticise directly. Yet the relative ease with which cars pass around the city must change quite dramatically if any real improvements in the quality of the urban environment are to be achieved. Politicians like Jon Rogers have the unenviable job of trying to persuade a population, wedded to a myth of personal freedom through car ownership, of this simple reality.