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Campaigners show London M4 widening plans will break law

3 March 2015

Campaign for Better Transport has demanded action after research exposed the illegal air pollution impacts of planned hard-shoulder running on the M4 through outer London and major commuter towns.

The Highways Agency is planning to turn the hard shoulder into an extra lane between Hayes and Reading (Junctions 3-12). Campaign for Better Transport looked at the details of the predicted air pollution impacts in the Highways Agency's own reports. This revealed how the extra traffic generated by the scheme would breach air pollution laws and mean large numbers of people would continue to be exposed to illegal air pollution [2].

Sian Berry, Sustainable Transport Campaigner at Campaign for Better Transport said

"People who live near this section of the M4 are exposed to dangerously dirty air that breaks legal limits. The UK Government has already been severely criticised in the courts for the slapdash way it approaches air pollution. The Highways Agency urgently needs to look again at this proposal and put public health above cramming yet more cars onto our roads."

The section of motorway runs through highly polluted areas including Heathrow Airport, Slough, Bray (near Maidenhead), Wokingham and Reading, all of which have been declared Air Quality Management Areas. The worst impacts found were new breaches of limits in Brentford near the A4, around Heathrow and next to Eton College, near Slough.

Campaign for Better Transport has called on the Highways Agency to amend its plans to, either:

1. Remove all-lane running from the proposals, reserving the hard shoulder only for congested times, as in the original Managed Motorway schemes, which still had beneficial effects on congestion. By not permanently widening the motorway, this is likely to remove a large part of the risk of creating induced traffic and worsening pollution.

2. Reduce the speed limit at all times to below 60 mph. This measure has been considered at other Smart Motorway locations and has been used to mitigate a similar breach of air pollution limits that would have been caused by the A556 Knutsford to Bowden scheme.


1. Campaign for Better Transport's response to the Highways Agency consultation on M4 junction 3-12 Smart Motorway scheme and statutory instrument on variable mandatory speed limits can be downloaded from our website.

2. The Highways Agency's proposals for the M4 are unlikely to be in compliance with air quality law. The air pollution impacts were revealed by examination of the full Preliminary Environmental Information (PEI) report, which the Highways Agency made available to Campaign for Better Transport on request. This includes clear indications that air pollution in areas currently above the legal limits, and expected to be below these limits in the 2022 'Do Minimum' scenario, will remain above the legal limit with the proposed scheme in place. If this is the case, then it is likely the scheme could not be pursued unless changes are made to the proposals.

3. A 2013 Supreme Court judgement ruled that the UK is failing in its legal duty to protect people from the effects of air pollution

In 2014, the European Court of Justice required the government to bring pollution within legal limits 'as soon as possible'

4. The Highways Agency has encountered significant problems with air pollution. for example, it has been forced to shelve plans for additional lanes on the M60 in Greater Manchester where air pollution is already above legal limits.

5. Campaign for Better Transport is the UK's leading authority on sustainable transport. We champion transport solutions that improve people's lives and reduce environmental damage. Our campaigns push innovative, practical policies at local and national levels. Campaign for Better Transport Charitable Trust is a registered charity (1101929).

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