Modelling Clean Air Zones
5 June 2018

Modelling Clean Air Zones

This article summarises the presentation given by David Connolly at Modelling World on 6 June 2018.

Air pollution is one of the most significant public health problems of our time. The Royal College of Physicians estimated in 2016 that around 40,000 premature deaths per annum in the UK are attributable to outdoor air pollution[1], more than 20 times those killed in road traffic accidents. In May 2018, the European Commission took the unprecedented decision to refer the UK, amongst other EU countries, to the European Court of Justice for failing to address chronic breaches of limits for nitrogen dioxide.

The issue of the air we breathe in our towns and cities is now firmly at the top of both public health and political agendas, with a range of measures being implemented and proposed to tackle road transport emissions. Clean Air Zones are one of the measures being taken forward, defined as areas “where targeted action is taken to improve air quality and resources are prioritised and coordinated in order to shape the urban environment in a way that delivers improved health benefits and supports economic growth”.

A Clean Air Zone (CAZ) aim to address all sources of transport-related pollution and reduce public exposure to it using a range of measures tailored to the location. They can be either Non-Charging or Charging, e.g. motorists must a charge to enter or move within a zone if they are driving a vehicle that does not meet the standard for that zone.

There are complex issues surrounding urban transport emissions, not least the dynamics of the relevant behavioural responses, which can be surprisingly difficult to untangle. SYSTRA is currently involved in CAZ feasibility studies in Southampton, Derby & Nottingham and Sheffield & Rotheringham, in which innovative transport modelling is being used to inform the development of these zones.

Our modelling work takes into account a range of factors, including the approach to modelling ‘Pay Once Per Day’ charges for non-compliant vehicles (typically pre-Euro 6 diesel or pre-Euro 4 petrol). In addition to ‘traditional’ driver responses of changes to route, destination and/or mode, Charging CAZs are also likely to affect vehicle ownership, as owners of non-compliant vehicles switch from diesel to petrol, to cleaner engine technologies and/or buy younger or cleaner vehicles.

The pressures to switch away from diesel are already affecting new vehicle sales, resulting in a drop in the sales of new (Euro 6) diesel vehicles, even though these vehicles would be unaffected by most of the charging regimes being considered in the various CAZ schemes in the UK. To put this into perspective, however, sales of new electric vehicles in the UK reached 2.3% this year; growing but still small, especially compared to their uptake in some of our neighbouring countries.

Of particular interest is how the frequency of driving into or through a proposed CAZ will affect vehicle replacement decisions. We can analyse long-running Automatic Number Plate Recognition datasets across an area, which sheds light on the distribution of trip frequency, from ‘once per year’ to ‘almost daily’, by vehicle type (including private cars, private hire vehicles, hackney cabs, light goods and heavy goods vehicles). Understanding these trip frequency distributions is crucial for predicting the total number of vehicles which need to be replaced to achieve the required reduction in emissions and hence to appraise the cost of the various potential Charging CAZ schemes being considered.

Modelling is just one element underpinning the development and implementation of CAZs (and Low Emission Zones), but with a robust and nuanced approach, it can give local authorities the confidence they need to proceed with making significant changes to the way vehicles (and therefore people) travel to and through their most socially, culturally and economically important areas.

Dr David Connolly is SYSTRA’s Director of Innovation and a leading expert in developing, appraising and delivering transport-related emissions schemes and strategies. For more information, contact


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