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WAQ76992 (e) Tabled on 08/08/2018

How will the effectiveness on environmental criteria of the extension of the 50mph speed limit on the M4 between J41 and J42 be monitored?

Answered by Cabinet Secretary for Economy and Transport Answered on 13/08/2018

The Welsh Government has a legal duty to ensure compliance with the Ambient Air Quality Directive.  Detailed investigations, including modelling using the approach set out in the Welsh Transport Appraisal Guidance (WelTAG) have been underway since October 2017 to establish those measures required to achieve compliance with the Directive a soon as possible.


The investigations and modelling undertaken to date have indicated that the implementation of 50mph speed limits along with other measures at five trunk road and motorway locations including the section of M4 west of Port Talbot have the potential to bring immediate improvements to air quality and bring forward compliance with the Directive.  These limits were implemented on a trial basis in June 2018.


The effect of the temporary speed limit is being monitored by roadside diffusion tubes which due to their proximity to the carriageway measure the level of NO2 from traffic rather than other sources.  The temporary speed limit will be kept in place as long as it is effective in improving air quality and necessary to assist in bringing down and maintaining NO2 levels below the Directive limit of 40 µg/m3.

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