Monday, 26 March 2018

Did London's congestion charge increase pollution?

There has been some news coverage today of a presentation at the Royal Economic Society's annual conference from , Professor John Heywood, Dr Maria Navarro and Professor Colin Green (Lancaster University, University of Wisconsin- Milwaukee and the Norwegian University of Science and Technology respectively, I believe) which allegedly claims that the central London congestion charge caused an unintended increase in some forms of pollution, namely nitrogen dioxide  (by 20%), although it reduced carbon monoxide, nitrous oxides and particulates.

The Sun reported this, but TLE (The London Economic) reports it in more detail. The Royal Economic Society (RES) has a press release about the presentation.  

The obvious simple reaction to this is to say the congestion charge "doesn't work" in terms of pollution.  However, it is much more nuanced than this and worthy of more detailed investigation.  Since I don't have a copy of the presentation, my comments are limited to what has been reported.

Reviewing the key findings

The report says that there were "significant reductions across a range of pollutants" in comparison to "comparison cities" and of course, because the congestion charge did reduce road traffic volumes and there were reports at the time of reduced emissions, it is a fair assumption that the net effects should be positive environmentally.   The key element of this research was to consider the composition of pollution.   What isn't clear is how this was actually measured.

The RES press release said:

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.

Improvement in bus services came about primarily because subsidies for bus franchises were increased so service frequencies could be increased, and some new routes introduced, and also the introduction of new bus lanes (and of course reduced congestion improved the ability to operate buses on existing roads).  The congestion charge helped raise revenue to support increased subsidies for buses, made reallocation of road space to bus lanes less disruptive for existing traffic and enabled some improvement in bus trip reliability.  

It continues: 

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).

This is true, although it's important to note that the mode-share for private car travel in central London was 12% in 2001 (before the charge was introduced).   The number of such trips dropped by around 33% between 2002 and 2006, but as a share of trips, it is still small.


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.

The evidence that reducing pollution is not just about reducing trips, but also reducing congestion (so the trips that remain operate more efficiently, and burn less fuel to travel the same amount of time).  This ought to have significant policy implications, because some environmentalists have treated traffic congestion as a tool to suppress car traffic, but congestion is not a positive.  It wastes time and energy, but also increases pollution.  Unfortunately, in recent policy debates about addressing air pollution in UK cities, the idea that policies that reduce congestion are worth pursuing seems to be ignored. 

Of course the study claims the NO2 increase of 10-20% is due to an increase in bus and taxi traffic.  TfL's own figures indicate around a 13% increase in the number of black cabs entering the charging zone between 2002 and 2006 (bear in mind black cabs** do not pay the congestion charge, neither do minicabs**.  There was also a 23% increase in bus trips, presumably almost all of that was due to TfL increasing services.

Traffic entering central London comparing 2002, 2003 and 2006 (Source: TfL)

From that there come a series of statements, which I think are worthy of questioning.

The authors argue that this increase is likely to reflect the shift towards diesel-based transport.
Thus, the congestion charge may have actually increased the harm from pollution.
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.

Is this analysis damning of congestion charging as a policy from an environment perspective?

Of course, without having access to the full paper, it is difficult to be certain of my hypothesis here, but this is what appears to have happened.

1. The congestion charge co-incided with policies to increase bus services.
2. Black cabs, being exempt from the congestion charge, were able to undertake more trips under conditions of lower congestion than before.
3. With more buses (a deliberate policy initiative) and more black cabs (because more trips were easier and there was no congestion charge applying),  NO2 emissions increased.
3. Ergo "the congestion charge increased pollution".

This would appear to be an overly simplistic conclusion which could cause some policy makers or lobbyists to think congestion charging, per se, is not a good policy environmentally.  However, such a conclusion would be wrong.

What could London have done differently?

Politically, the congestion charge couldn't have been introduced without an exemption for black cabs, and the expansion of bus services was intended to offer an alternative for some private car trips (although it is unclear whether there was much of a modal shift from car to bus, as it is more likely the underground and rail services took some of the trips).  However, the congestion charge as a policy in itself was not specifically responsible for an increase in diesel vehicle trips and NO2 pollution.  

The congestion charge introduced a flat charge on private cars and goods vehicles (the latter saw a 13% decline in trips between 2002 and 2006), but did not charge buses nor taxis (of any kind).  So it is hardly surprising that while private car and goods traffic declined, the vehicles not subject to a charge increased in trips.  In other words, the congestion charge reduced pollution from the vehicles that it charged, but not the vehicles that it did not charge, in terms of NO2.  Bear in mind that the proportion of diesel cars in the UK vehicle fleet increased during this period (not least because of misguided changes to vehicle excise duty intended to reflect CO2 emissions only).

Technology has progressed since 2003.  30% of London buses are now hybrids, with 0.8% now zero-emission electric and fuel-cell vehicles. So a similar increase in bus services today would mean a lower increase in emissions, particularly if priority was given to putting new hybrid vehicles on central city routes.  However, although increasing the capacity and frequency of bus services is a logical complementary policy to congestion charging, and may be desirable (depending on the design of the charging scheme itself) it is not absolutely essential and certainly today, with the engine technology available (notwithstanding cities with trolley buses who can always claim zero emissions from such vehicles), it does not necessarily follow that more buses means higher NO2 emissions.   I suspect that had London not increased bus capacity (by increased frequencies, but rather taken the gains from reduced congestion to improve trip reliability), although the congestion charge itself may have been marginally less effective, NO2 would not have increased as was reported.  

For black cabs it is simpler, as there isn't a compelling policy reason why people who choose to travel by taxi into or around central London should be preferred over those who bring their own car.   Bear in mind that black cabs circulate "empty" searching for fares, contributing to both congestion and pollution, whereas a private vehicle always undertakes a "useful trip" in transport utility terms and when it isn't, it is not polluting or contributing to congestion.  Stockholm, Gothenburg and Singapore do not make any taxis exempt from their urban congestion charging schemes. Transport for London has been consulting on removing the exemption for minicabs (but not black cabs).   To be fair, had black cabs not been made exempt, they would have opposed the congestion charge and been a problem for the Mayor in introducing the charge.  However, if the exemption were removed, it is not clear how much of a difference it would make with the current charge structure.

The congestion charge is set at £10.50 (US$14.93) for unlimited trips within the zone, for vehicles registered with Autopay (which means vehicles are automatically charged when detected entering or circulating within the zone, rather than having to consciously pay for a trip each day in a separate transaction).  It seems likely that most black cabs (and minicabs) would absorb this in their fares and spread that cost, although there will probably be a small incremental reduction in vehicles.


The congestion charge did NOT increase pollution, in fact the report quoted states that with the exception of NO2, other pollutants reduced.  However, given the information presented, it is not the congestion charge that is to blame. It is the increase in bus services, approved by the Mayor of London (using the engine technology of the time) and the increase in black cab trips (exempt from the congestion charge) that appears to be the evidence for a causative relationship in increasing NO2 pollution being attributed to the congestion charge.

It would be highly misleading to draw the conclusion that introducing congestion charging today or expanding it would have the same result.  Hybrid or electric buses can mean bus services can be increased without the same result, and taxis (both kinds) need not be exempt at all (they aren't in other jurisdictions).

* The press release stated the City of London introduced the congestion charge, but it was Transport for London (as the area charge crosses the boundaries of several other boroughs, most notably Westminster, but also Camden, Southwark, Lambeth, Islington, Tower Hamlets and Hackney).

** For those unfamiliar with the split in the taxi industry in the UK: Minicabs (legally "private hire vehicles") tend to offer fixed fares, are not metered and are only allowed to pick up passengers on pre-booked trips (which may be by phone, online or by asking at a minicab office, in which case a minicab may be available immediately). Black cabs (legally "licensed hackney carriages") are metered and are legally entitled to pick up "hailing" passengers (people who flag down a taxi in the street or request a trip from a taxi rank).  Black cabs can also offer fixed fares to passengers and be pre-booked, just like minicabs, but the licencing requirements for black cabs are stricter.  Both types of taxi are licensed by local authorities (but in London by Transport for London).

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