27 April 2011

The London Congestion Charging Scheme (CCS)—which charged for travel into central London and reduced traffic volume—showed little evidence that it improved air quality, according to a new study published today by the Health Effects Institute (HEI). The study, HEI Research Report 155, “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 improve air quality as well. Kelly and his team explored the impact of the Congestion Charging Scheme on air quality through 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.

Despite their comprehensive efforts, 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. 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,” said Dan Greenbaum, HEI’s President.

Source: The Health Effects Institute