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HEI study finds London Congestion Charging Scheme shows little evidence of improving air quality

The London Congestion Charging Scheme (CCS)—which charged for travel into central London and reduced traffic volume (earlier post)—has shown little evidence that it improved air quality as well, according to Part I of a new study published by the Health Effects Institute (HEI). The study, “The Impact of the Congestion Charging Scheme on Air Quality in London”, was led by Professor Frank Kelly of King’s College London as part of HEI’s research program to measure the possible health outcomes associated with actions taken to improve air quality.

Although the London CCS was designed to improve traffic and not necessarily air quality, early projections had suggested it could also improve air quality. Kelly and his team used a multifaceted approach to explore the impact of the Congestion Charging Scheme on air quality: a variety of emissions and exposure modeling techniques, analysis of air monitoring data, and a newly developed assay for the oxidative potential of particulate matter collected on filters at urban background and roadside monitors. Part I of the report deals with emissions modelling and analysis; Part II, to be released next month, analyzes the oxidative potential of PM.

The CCS offered an unusual opportunity to investigate the potential impact on air quality of a discrete and well-defined intervention to reduce traffic congestion in the middle of a major city. The CCS was implemented in London in February 2003 with the primary aim of reducing traffic congestion by charging vehicles to enter the central part of London, defined as the congestion charging zone (CCZ).

In an earlier study based on data from the first year of the scheme, members of the investigative team had reported early findings of modest reductions in the number of vehicles entering the zone and had projected declines of about 12% in emissions of both PM10 (particulate matter with an aerodynamic diameter of ≤10 µm) and nitrogen oxides (NOx) within the CCZ.

—Kelly et al.

The investigators did not find consistent evidence of improved air quality resulting from the CCS. In part it is difficult to identify significant air quality improvements from a specific program—especially one targeted at a small area within a large city—against the backdrop of broader regional pollutant and weather changes.

Within the CCZ, the investigators projected a net decline of 1.7 ppb in the annual average mean NOx concentration and a decline of 0.8 µg/m3 in PM10. The modeling also suggested that a major proportion of PM10 might be accounted for by regional background levels, but that contributions from tire and brake wear might also be important. NO2 was projected to increase slightly, by 0.3 ppb on average; the investigators attributed this increase to higher NO2 emissions associated with the introduction of particle traps on diesel buses as part of Transport for London’s improvements in the public transport system.

From their comparison of actual air pollutant measurements within the CCZ with those at control sites in Outer London, the investigators reported little evidence of CCS-related changes in pollutant levels at roadside monitoring sites, where their modeling had suggested the most pronounced effects would be seen.

—Kelly et al.

Also, some behavioral adjustments among the population, e.g. increased diesel-powered taxi and bus trips to transport people into the zone, may have offset any benefits. Finally, other changes occurring at the same time (e.g. the introduction of more filter-equipped diesel buses in response to a separate rule) likely also affected air quality and obscured effects of the CCS.

The Congestion Charging Scheme was one of the first to be implemented in a major city in Europe or the US—and did show measurable reductions in traffic volume—but air pollution does not know precise boundaries so any benefit of the CCS or air quality appears to have been lost in the larger regional pollution mix.

—Dan Greenbaum, HEI’s President

Overall, HEI’s Review Committee concluded that Kelly and colleagues’ investigation represents a creative effort to explore a subtle change in air quality associated with a complex intervention to reduce traffic congestion. Although they were unable to demonstrate a clear effect of the CCS either on individual air pollutant levels or on oxidative potential of particulate matter, their study offers lessons for future studies of interventions that are expected to influence air quality.

The London Congestion Charging Scheme was a world leading traffic intervention aimed at controlling excessive vehicle flows in central London. The findings reported in this HEI study will hopefully be of use to other administrations considering introducing traffic management schemes so that they can achieve vehicle reductions as well as improving air quality at the same time.

—Professor Frank Kelly

In addition to the Investigators’ Report by Kelly et al., Research Report 155 includes a Commentary by HEI’s Health Review Committee, which summarizes its independent review of the study and an HEI Statement that provides a nontechnical summary of the study and the committee’s comments.

The Health Effects Institute is an independent, non-profit research institute funded jointly by government and industry to provide credible, high-quality science on air pollution and health for air quality decisions. Typically, HEI receives half of its core funds from the US Environmental Protection Agency and half from the worldwide motor vehicle industry. Other public and private organizations periodically support special projects or certain research programs.

HEI has funded more than 280 research projects in North America, Europe, Asia, and Latin America, the results of which have informed decisions regarding carbon monoxide, air toxics, nitrogen oxides, diesel exhaust, ozone, particulate matter, and other pollutants. These results have appeared in the peer-reviewed literature and in more than 200 comprehensive reports published by HEI.




It may not a matter of reduced pollution but rather a way to stop further increases.


If you want to know the real reason, since charging was introduced the area has ben subjected to tinkering with traffic lights to increase 'red' times and a number of road-space robbing initiatives which have engineered in congestion. Not surprisingly, whilst traffic levels have fallen, congestion has got worse, hence a rise in air pollution .

This is New Realism (transport theory) working at its best (or worse)!

Ref: Mannings (2006)'Traffic and Roads since 2000: Policy, Politics and Perceptions of Progress', Local Government Studies, 32(3), 273-292

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