The impact of congestion charging on vehicle emissions in London

The London congestion charging scheme (CCS) was successfully implemented in February 2003 and has measurably reduced traffic flows in central London. The air pollution impact of the scheme has been difficult to assess using ambient measurements alone as the air pollution concentrations in 2003 were higher than in 2002 because of unusual meteorological conditions. However, a comprehensive analysis of the impact using detailed traffic data, combined with the Environmental Research Group's road traffic emissions model, has identified a number of important results. First, between 2002 and 2003, total NOX emissions in the charging zone have reduced by −12.0% ±12% (2σ) and have increased on the inner ring road (IRR) by +1.5%. PM10 emissions have reduced by −11.9% in the charging zone and by −1.4% on the IRR. There is a significant reduction in the emissions of NOX and PM10 associated with increases in vehicle speed and that this is as important in reducing emissions as changes in vehicle numbers. There is also evidence that the speed changes in km h−1 are uniform across the whole range of average speed and therefore changes at the slower speeds have a disproportionate effect on vehicle emissions. Second, that changes in vehicle km, as a result of the scheme, are large (−15% ±4% (2σ)) particularly in the charging zone itself. To meet the demand to travel into central London there has been increased bus use. However, the expected increase in emissions from buses have been mostly offset by the widespread introduction of particle traps to the new and existing bus fleet as well as the introduction of newer technology bus engines. Finally, there is a reduction in emissions of CO2 (−19.5%) but that unlike NOX and PM10 little additional benefit is apparent through new vehicle technology. The evidence presented shows that the congestion charging schemes could assist in attaining both the UK government's targets on air pollution as well as those relating to climate change and other international obligations.