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Further information about reducing speed limits to 50mph to reduce nitrogen dioxide (NO2) pollution.

First published:
21 December 2021
Last updated:

Benefits of lower speed limits

We want to improve air quality for the people of Wales as soon as possible. Vehicle emissions are a major contributor to nitrogen dioxide (NO2) levels at roadside locations. Where levels of NO2 are above statutory limits, we must take action to reduce concentrations as soon as possible, including consultation with the public on proposed mitigations.

Breathing air with a high level 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.

Air quality monitoring networks are located across the UK and feed into the Department for Environment, Food and Rural Affairs (DEFRA) Pollution Climate Mapping (PCM) models which have been designed to assess compliance. 

We have a legal duty to comply with the Air Quality Standards (Wales) Regulations 2010, which sets an annual mean limit value for NOof 40 μg/m3, and an hourly limit of 200ug/m3, not to be exceeded more than 18 times in a calendar year. Through PCM, levels of NO2 have been identified as being above the legal limit at several locations on the Welsh Government’s motorway and trunk road network. We must act to improve public health.

Enforcing lower speed limits

The 50mph speed limit is being enforced in 5 locations on the motorway and trunk road network using average speed cameras.

Image of how air quality cameras look on the road
Image of how air quality cameras look on the road

This is to make sure that drivers stick to the speed limit 24/7. The cameras were installed during August 2019 for monitoring purposes.

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

An air quality camera with green casing.
An air quality camera with green casing.​​

While most people drive at around 50mph, the National Police Chiefs Council (NPCC) suggests that many people are still driving at 57mph (10% + 2mph over the limit).

It’s essential that drivers stick to the speed limit to help us to improve the air quality, so we will be issuing advisory notices from autumn 2021. However, the police and GoSafe will still enforce the speed limits when considered necessary. GoSafe is the Wales Road Casualty Reduction Partnership.

Low emission vehicles

The 50mph speed limit applies to all vehicles. We acknowledge that low and ultra-low emission vehicles are likely to emit much less (if any) NO2, but having a separate speed limit for these vehicles would make the road more dangerous and reduce the overall effectiveness of the 50mph speed limit.

Measuring NO2 levels

We assess air quality using diffusion tubes and continuous monitors. Diffusion tubes measure ambient NO2 over a one month period. The continuous monitors provide real-time data.

The data we record will show us if we’re meeting our target NO2 levels.

We have published several reports that includes the data recorded at 5 locations where levels of NO2 have exceeded legal limits. These reports can be found in the documents section.

Welsh Government will publish a 2022 update to the supplemental plan and consultation on the draft document will be undertaken during the spring. The update will include the full air quality and traffic data up to 2021.

The first report, published in October 2019 provided interim direction on the possible effectiveness of the speed limits, comparing the full 12-months’ worth of data following their initial implementation on a temporary basis in June 2018 against the 6-months’ worth of data prior. The report also provided an update on compliance with the speed limits following their implementation.

The second report, published in March 2020, provided further information on the speed limits and compared full calendar year data recorded for 2018 against data recorded during the 2019 calendar year. The report again provided an update on speed limit compliance, including data recorded following provision of the average speed cameras. 

We are considering which additional measures might be appropriate to help us to meet the NO2 targets.

Introducing 50mph speed limits in other areas

The Pollution Climate Mapping model (PCM) is designed to meet part of the UK's legal requirements to report on the concentrations of particular pollutants in the atmosphere and is the agreed national source for identifying areas or routes where limits are exceeded. In 2017, the model identified 5 roads where NO2 levels were higher than the limit.

Two further locations have since been identified where the NO2 levels are higher than they should be, using the annual PCM model. These are:

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

We are carrying out WelTAG investigations to help identify the most suitable actions to ensure that NO2 levels at these locations drop below the limit in the shortest possible time. Welsh transport appraisal guidance (WelTAG) is a framework that we use to develop transport systems. It was developed to make sure that we meet a number of objectives and to show how we will use public money.

It is possible that measures including lower speed limits may be introduced elsewhere, but only if the investigations and modelling show that they are needed.

Choosing a 50mph speed limit

The 50mph speed limit was based on the fact that speed emission curves for vehicles indicate that 50mph is likely to be the speed at which vehicles emit the lowest level of NOx.

Reducing pollution

There are lots of different types of pollution. The 50mph speed limit helps us to tackle nitrogen dioxide (NO2) pollution produced by road vehicles. Reducing the speed limits will reduce your exposure to NO2. NOcan cause lung disease and asthma.

We are also tackling pollution caused by industry and domestic burning.

Reduced NO2 levels during the Coronavirus (COVID-19) pandemic

We have recorded that NO2 levels have gone down since we introduced the 50mph limits in 2018. 

We know that people weren’t able to travel during the pandemic as much as they did before, so NO2 levels were lower. It shows that we can lower NO2 levels if we choose different ways to travel like walking, cycling and using public transport.

50mph speed limit trial

We worked with GoSafe and the police to explore different ways to make the speed limits work before introducing fines.

This included letting drivers know where and why the limits are in place. We are now taking action to deal with the most serious offenders. This will help us to achieve the reductions we need to make in the shortest possible time.

50mph speed limits on busy roads

Lowering the speed limit on busy roads means that, when traffic is free flowing, we’ll emit less NO2.

More information

The documents referred to in this page are available to view on the Welsh Government website via the following links:

Contact us

For more information, contact: