Boston, Massachusetts – A program that charged drivers travelling into central London to reduce traffic volume has shown little evidence of air quality improvement, according to a new study published by the Health Affects Institute (HEI).

The London Congestion Charging Scheme (CCS) was designed to improve traffic and not necessarily air quality, but early projections had suggested it could improve pollution levels as well. The research team used a variety of emissions and exposure modelling techniques, analysis of air monitoring data, and a study of particulate matter collected on filters at urban background and roadside monitors. The team did not find consistent evidence of improved air quality resulting from the CCS.

The researchers said that, in part, it is difficult to identify significant air quality improvements from a specific program, especially one targeting a small area within a large city, against a backdrop of broader regional pollutant and weather changes. Behavioural adjustment among the population, such as increased diesel-powered taxi and bus trips to transport people into the zone, may have offset any benefits, and other changes occurring at the same time, including the introduction of more filter-equipped diesel buses in response to a separate rule, likely also affected air quality and obscured the effects of the CCS.

“The Congestion Charging Scheme was one of the first to be implemented in a major city in Europe or the U.S., 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, president of HEI.

Although the researchers were unable to demonstrate a clear effect of the CCS on individual air pollutant levels or on oxidative potential of particulate matter, the study offers many lessons for future studies of interventions that are expected to influence air quality, the HEI said.

“The London Congestion Charging Scheme was a world leading traffic intervention aimed at controlling excessive vehicle flows in central London,” said Professor Frank Kelly of the Environmental Research Group at King’s College London. “The findings reported in this HEI study will hopefully be of use to other administrations considering introducing traffic management schemes so they can achieve vehicle reductions as well as improving air quality at the same time.”

Connect with