Given the lack of progress by Portsmouth City Council in addressing air pollution, I began wondering how a Clean Air Zone (CAZ) might be implemented in the city and what effect it might have. There are several types of Clean Air Zones but most interest has been in charging older vehicles that enter busy areas. Since less polluting vehicles attract lesser or no charges, this intended to encourage a shift to less polluting vehicles. This has an impact on residents and businesses that have to replace their vehicles. The revenue from charges is typically invested in sustainable transport.
Arguably, this approach does not go far enough. As even the Portsmouth transport cabinet member says, “We simply have too many vehicles in this city“. Punitive prices could be used to quickly force a change in transport usage, but this is hardly going to be popular.
It is important to remember that our current transport situation is unsustainable. A significant chunk of pollution is created by diesel cars. To comply with air quality law, these vehicles need to be replaced. The question is how we go about doing that in a fair manner considering many people can barely afford the essentials. Studies have found that the economic benefits to health may outweigh the cost of updating vehicles. The necessity to act has already been established in the High Court and for the benefit of public health.
Predicting the impact of CAZ is complicated because people may switch to other forms of transport or cancel their trip completely. Local and national government often use computer models to try and predict the impact on air quality. Alternatively, we can look at how other cities have implemented CAZs however other air quality initiatives and different transport usages complicate the picture. DEFRA has presented a rosy picture of the effectiveness of CAZ. According to a 2017 DEFRA technical report:
From the options considered, establishing Clean Air Zones (CAZs) is the most effective way to bring the UK into compliance with NO2 concentration levels in the shortest possible time (Table Ex.3). […] CAZs are predicted to have their greatest impact on air quality in the year of implementation […], leading to an 18% reduction in NO2 concentrations. […] These projections have been produced from the SL-PCM model.
Birmingham has been told to implement a CAZ. Their projections are slightly less optimistic with a 9% NO2 reduction for a CAZ covering Buses, coaches, taxis, HGVs and LGVs (in jargon, a “class C-CAZ”), up to a 13% NO2 reduction in a CAZ that also covers private cars (“a class D-CAZ”). These are smaller but still significant improvements. However, these predictions don’t seem to match the experience cities that have already implemented similar zones. According to other DEFRA reports, CAZ projects around Europe have been effective in reducing particulate pollution but have a limited impact on NO2 levels. Germany has been most successful in reducing NO2 pollution using CAZ probably because their schemes include private cars. Outside of Germany, most CAZ schemes ignore private cars and have generally seen no NO2 improvements. Lisbon is also an exception which has reduced NO2 by 1-7% because they including private cars in their CAZ scheme. According to a review of CAZ schemes:
In German cities reductions in annual mean PM10 and NO2 concentrations up to 7% and 4% respectively due to the implementation of an LEZ have been reported. […] In other countries the picture is much more mixed with no effects [on NO2] generally being observed. This may be explained by the German LEZs restricting passenger cars, particularly diesel cars as well as HDVs.[Heavy Duty Vehicles …] Many of the studies, however, have used simple statistical methods that have not taken sufficient account of the confounding factors that affect urban air quality.
This finding is repeated in the London Low Emission Zone, which did not include cars in their scheme and have seen no appreciable drop in NO2. (London has recently started charging older vehicles which might help.) Similar results were seen in the Netherlands. A study of CAZ in Beijing, which only included light duty goods vehicles, predicted a fairly minor drop of 2% in NO2 levels.
In summary, Clean Air Zones are only effective in tackling NO2 levels if private cars are included in the charging scheme. This targets diesel cars, which are a major source of pollution in cities. Even if private cars are included, the impact of a charging CAZ may be less than predicted by current modelling. This might be due to different vehicle usage in the UK compared to European cities, but I doubt that is significant enough to account for the discrepancy. Portsmouth needs about a 10% drop in NO2 to come into legal compliance.
Given that we need a strong tool to improve air quality, we can look at cities that have banned older diesel vehicles. Older diesel bans are being introduced in some London neighbourhoods, as well as several German cities. We should watch their progress closely to see if improvements can be achieved elsewhere.