London's congestion charging scheme (CCS), which charges motorists for entering central London to try to cut traffic volume, has so far shown little evidence of improving air quality, scientists said.
A study led by researchers at King's College London found that the charge, introduced in 2003 and the first such levy in a major city in Europe or the US, had indeed helped reduced traffic in the city centre.
"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, president of the United States-based Health Effects Institute (HEI), which published the study.
Frank Kelly from the environmental research group at King's College, who led the study, analysed a variety of emissions using a range of exposure modelling techniques. His group also studied air monitoring data, and used a newly developed test which assesses the oxidative potential of particles collected on filters at urban background and roadside monitors.
No improved air quality
Despite this wide range of different tests, the researchers did not find consistent evidence of improved air quality as a result of the congestion charge, they wrote in their study.
They noted that it is hard to identify significant air quality improvements from a specific project, particularly one targeted at a small area in a large city, against the backdrop of broader regional pollution and weather changes.
They also said that other changes, such as increased used of diesel-powered taxi and bus trips to transport people in and out of the congestion charge zone, may have offset any benefits.
Kelly said London's congestion charge was "a world leading traffic intervention" and said he hoped his findings would 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."
All profits made from the congestion charge have to be ploughed back into upgrading London's rather aging and often overcrowded transport system.
(Reuters Health, April 2011)