How have the CAZ boundaries been set?

We are considering a CAZ as part of our Clean Air Plan, though a decision to have a CAZ has not yet been made.

The size of a potential CAZ has not yet been confirmed. We are currently testing two possible boundaries – a “Medium” and a “Small” – please see the map showing the indicative boundaries which will be used in the modelling. The modelling will determine the optimum size needed to achieve compliance with air quality standards in the shortest time possible.

It is important to note that just because an area is not included within the CAZ boundary, it does not mean that air quality is of a lower priority here, or that the air is currently clean in that area. Although the CAZ is geographically defined, a CAZ will have the effect of cleaning up the vehicle fleet across the whole city and region because polluting vehicles will be charged to drive in those areas where there is the most demand to travel (the central area) and hence people will change their vehicle sooner than they otherwise would, or use a cleaner mode (if we decide to charge cars). The CAZ will also deter “through” trips for polluting vehicles, again improving air quality on the arterial routes that pass through areas that are not within the bounds of the CAZ.

We are also assessing, through traffic modelling, the impact of displacement, i.e. the extent to which drivers will try to avoid the zone by driving round it – and possibly increasing pollution in the non CAZ areas that border the CAZ. The medium zone reduces displacement in comparison with the small because it is further to drive to avoid the zone.

The boundaries of the CAZ are determined by assessing its desired effect on driver behaviour rather than purely ambient air quality levels.

In summary, just because the CAZ boundary does not include an area, it does not follow that the area will not benefit from the CAZ. We intend to ensure that everywhere within the city complies with the EU standards for pollution in the shortest time possible.