The aim of the CAZ is to reduce NO2 concentrations within the city rather than reduce traffic levels. However, the CAZ may alter traffic from the present levels in the short term, as people adjust their travel choices. The difference in traffic would depend on what type of CAZ is implemented i.e. there is likely to be more of an impact on traffic if all vehicles including cars (Class D) were charged compared to a Class A being charged. Potentially, there will be a reduction in traffic on the roads within and approaching the CAZ as not everyone will have a compliant vehicle and/or be willing to pay the charge.
However, any congestion improvements are likely to be short term only. As vehicles are replaced with cleaner models and can drive within the zone without charge, traffic levels are likely to return to current levels or change in line with predictions.