The reduction in traffic congestion associated with the introduction of a charge for driving during prime hours in London''s central district reduced some forms of pollution: carbon monoxide, particulate matter and nitrous oxide. But it has had the unintended consequence of increasing more damaging forms of pollution – NO2 emissions – which are likely to reflect the shift towards diesel-based transport.
These are the findings of research by Professor Colin Green, Professor John Heywood and Dr Maria Navarro, to be presented at the Royal Economic Society''s annual conference at the University of Sussex in Brighton in March 2018. Their study indicates that the congestion charge may have actually increased the harm from pollution, as NO2 is linked to particularly adverse health outcomes including severe lung and respiratory problems.
Starting in 2003 the City of London imposed a charge for driving during prime hours in its central district. While aimed at reducing traffic congestion, reductions in air pollution were championed as an additional major benefit.
Whether or not this additional benefit came to fruition takes on increasing importance as a Parliament select committee recently declared London air pollution a public health emergency and demanded new charges within the congestion zone specifically designed to combat vehicle emissions.
With as many as 50 thousand premature deaths in the UK due to air pollution and with automobile exhaust the single most rapidly rising source of deaths world-wide, the time is ripe for understanding the consequences of the original London congestion charge on air pollution.
As the congestion charge has reduced traffic flows, a reduction in pollution might be anticipated. Indeed, research comparing pollution in Central London before and after the introduction of the charge finds reductions in overall pollution levels. Yet this simple comparison may not be useful as the reduction could have happened even without the charge.
The new study adopts a variety of approaches that involve using the most populous 20 cities in Britain, not including London, as comparison groups.
In addition, examining aggregate pollution levels will be likely to miss important changes in the composition of pollution. While some trips to central London simply may not take place, congestion charging policies seem more likely to change the method of transit. The charge made driving in London more expensive and generated improvements in bus services.
In addition, certain forms of transit were exempt from the London congestion charge including bikes, motorcycles, taxis and mass transit. As might be anticipated, more travellers used buses and taxis in central London. This caused a move away from predominantly petroleum-based transport (private vehicles), towards diesel based transport (black cabs and buses).
The UK Department of Energy and Climate Change concludes that fumes from diesel are significantly more harmful than those from petrol engines and the World Health Organization lists diesel but not petrol fumes as a Group 1 carcinogen. Thus, even as overall pollution goes down, the harm from that pollution could increase.
The new study demonstrates reductions in three traditional pollutants: carbon monoxide (CO), particulate matter (PM10) and nitrous oxide (NO). These reductions are as large as 25% to 30% for PM10 and NO.
The researchers show that the reduced pollution per mile travelled in the zone exceeds that expected from the reduction in traffic flows alone. Thus, the reduction in these pollutants reflects, in part, removing very high levels of traffic congestion as well as reducing miles driven.
At the same time, the study demonstrates a sharp increase in NO2 emissions, in the range of 10% to 20%. The authors argue that this increase is likely to reflect the shift towards diesel-based transport.
Importantly, NO2 is linked to particularly adverse health outcomes including severe lung and respiratory problems. Moreover, the scientific consensus increasingly regards the association between respiratory morbidity and NO2 to be causal and not just a function of other associated pollutants.
Thus, the congestion charge may have actually increased the harm from pollution. In additional tests concerned about making proper inference, the researchers find that the most robust of all their results is the clear increase in NO2.
They conclude that the reduction in congestion associated with charge simultaneously reduced some forms of pollution but had the unintended consequence of increasing more damaging forms of pollution.
Did the London Congestion Charge Reduce Pollution? – Professor Colin Green (Norwegian University of Science and Technology), Professor John Heywood (University of Wisconsin – Milwaukee) and Dr Maria Navarro (Lancaster University)