Environmental Research Group
Monitoring » Case Studies
The Effects of the London Congestion Charging Scheme on Air Quality
The Environmental Research Group has recently completed an extensive three-year study into the effects of
the London Congestion Charging Scheme on air quality, funded by the Health Effects Institute in the US.
The project brought together data analysts, modellers, epidemiologists and statisticians from King's College London,
London School of Hygiene & Tropical Medicine and St George's, University of London.
This team is also researching the effects of the London Low Emissions Zone on air quality and health,
funded by the HEI and Transport for London.
The London Congestion Charging Scheme (CCS) is a traffic management scheme introduced in February 2003 with the
aim of reducing the numbers of vehicles entering central London during the working week, thereby reducing congestion.
The initial charging zone incorporated approximately 22 km2 of central London, or 1.4% of the Greater London
area and contained some of the most congested conditions in the capital. After one year of operation,
traffic entering the charging zone during charging hours had reduced by 18%. Recent results from Transport
for London (TfL) illustrate that such a reduction continues to be maintained: in relation to pre-charging
conditions in 2002, traffic entering the CCZ during 2006 was 21% lower (TfL 2007). In contrast to the
within-zone findings, traffic on the inner ring road (the boundary of the congestion charging zone along
which no charge is applied) has remained similar to levels before the introduction of charging.
The overall objective of this retrospective study was to assess whether the reduction in congestion and traffic
in London, achieved following the introduction of the scheme, has had an impact on air quality in the
Capital. The outcome of this work has the potential to both inform future decisions and provide an
analytical framework for similar road pricing schemes that are introduced in other cities around the world.
This study developed methods to evaluate the possible effects of this scheme on air quality by comparing pre
and post CCS periods within and outside the congestion charging zone (CCZ). The three main components to
the study included (a) an estimation, using emissions modelling, of the likely nature and scale of the
air quality changes; (b) the setting up and analysis of a database of air pollution measurements
using a range of approaches; and (c) analysing PM filters for oxidative potential.
In principle the CCS, in reducing congestion and the numbers of vehicles crossing the cordon,
should reduce emissions and improve air quality both inside and outside the congestion charging zone (CCZ).
However, this is an unrealistically simple assumption and a number of issues need to be considered,
including vehicle speed, flow and fleet composition. Furthermore, whether the CCS results in observable
changes in air quality in London, depends on factors such as the magnitude of any changes in emissions,
vehicle flows and/or speeds, the location of air quality monitoring sites and underlying changes
in London’s air quality that may act to mask or accentuate any effect of the CCS.
This assessment of the impact of the CCS on Londo's air quality included a detailed comparative emissions modelling exercise.
Estimates of emissions inside and outside the CCZ the impact of the CCS were modelled to provide a comparator for the outcomes
of the subsequent analyses of air quality measurements made before and after introduction of the CCS.
An air pollution database of over 14 million measurements covering the period February 2001 to February 2005 was also
assembled. This dataset was used to investigate changes in air quality associated with the introduction
of the CCS through a range of statistical approaches specifically developed for the study.
In addition to possibly altering the ambient concentration of PM in London, introduction of the CCS may have influenced the
nature of the ambient PM mix owing to associated changes in traffic densities, speeds and the vehicle mix (hence fuel use).
Work was undertaken to examine the oxidative properties of London PM to ascertain whether the implementation of the CCS
had resulted in a change in its toxicity. To achieve this, we utilised a screening approach to access the oxidative
potential of small quantities of PM recovered from collection filters to be assessed. The method, based on antioxidant
depletion from synthetic lung lining fluid, is able to replicate events occurring in vivo following PM inhalation.
The final study report was submitted to the Health Effects Institute in Autumn 2007 and is currently under review.
A summary of results and conclusions will be posted here once this report has been finalised and released by the
funding body. The full report will also be available for downloading on this web site.
Item Date: 17/01/2008
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