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We have reduced speed limits to 50mph to reduce nitrogen dioxide (NO2) pollution.

First published:
22 June 2018
Last updated:

What we are doing

We have brought in 50mph speed limits at 5 locations across Wales and are taking action to ensure that drivers comply with these limits.

Why we are doing it

We have a legal duty to comply with air pollutant limits for the protection of human health. Outdoor air pollution is recognised by the World Health Organisation as the biggest environmental risk to health.

Vehicle emissions are a major cause of NO2 levels by roads. Regular assessment has identified NO2 levels above the legal limit at several locations on the motorway and trunk road network. We must take action to reduce roadside pollution at these locations.

We understand that speed limits may be unpopular. However, we know they are likely to be the quickest and most effective way to reduce NO2 levels.

How does NO2 levels impact our health?

Breathing air with high levels of NO2 can:

  • irritate your airways
  • worsen diseases like asthma
  • cause coughing, wheezing or make it difficult for you to breathe
  • cause you to develop serious illnesses like heart disease, lung cancer and asthma
  • increase your risk of catching respiratory infections.

Why 50mph?

The reduced speed limit is based on research for light duty vehicles that indicates emissions are lowest at speeds up to 50mph.

What we have done so far

  • Reduced some speed limits to 50mph.
  • Added new signs to highlight the speed limits.
  • Added average speed cameras to monitor traffic speeds.

This affected the following roads:

  • A483 between junctions 5 and 6 Wrexham
  • A494 between the Wales/England border and St David’s Interchange Deeside
  • A470 between Upper Boat and Pontypridd
  • M4 between junctions 41 and 42 Port Talbot
  • M4 between junctions 25 and 26 Newport


Drivers must stick to the speed limit to help us improve air quality.

While most people are complying with the reduced speed limit the National Police Chiefs Council (NPCC) suggests that there are still people driving at higher speeds.

To address this, the 50mph speed limit is being enforced in 5 locations on motorways and trunk roads with average speed cameras.

The cameras are green, rather than the standard yellow, to show that they are measuring speeds for air quality purposes.

Measuring pollution

We assess air quality using several different methods on an ongoing basis. The data shows us if we’re meeting legal NO2 limits.

We publish reports that includes the data recorded where levels of NO2 have exceeded legal limits. These reports are linked from the more information section below.

We will publish a 2023 update to our Air Quality Plan and will consult on proposals for action at two further locations on the motorway and trunk road network.

We are considering what other measures might help us to ensure sustained compliance with NO2 limits.

Introducing 50mph speed limits in other areas

Ongoing assessment has shown two more locations where NO2 levels are too high. These are:

  • A470 Coryton Interchange to Nantgarw Interchange
  • M4 junction 43 Llandarcy to M4 junction 44 Lon Las

We are investigating actions that may be taken to support urgent compliance at these locations also. 

This may include lower speed limits if evidence supports their introduction here too.

More information

More detailed information about how we are tackling air quality around roads can be found in these documents:

Contact us

For more information, contact: