Air quality library

This digital archive holds a collection of reports divided into the following categories: 

• Annual evaluation reports – providing our annual review into air quality alongside our network. 
• Assessing air quality alongside the Strategic Road Network (SRN) – providing a review of each of our road links identified as being above the air quality thresholds based on government data.  
• Monitoring – providing all of National Highways monitoring data.  
• Research – providing details of all the research we have completed to try and improve air quality alongside our network.  

If you need help finding any data, interpreting results or require more detailed information, please contact us at: airquality@nationalhighways.co.uk 

Annual evaluation reports

We complete an annual review of each SRN Pollution Climate Mapping (PCM) link identified as non-compliant. These annual evaluation reports provide a summary of the compliance for all sections of the SRN that have been assessed during that year. They also provide an update on the progress of any measures that have been implemented to improve air quality on these links during this time.  

Each report includes:  

• a review and summary of each link.  
• the compliance status of each link. 
• the year the link became compliant or is predicted to come into compliance.  
• an audit summary report for each link, outlining measures to improve air quality, where applicable.  
• monitoring data for each link, where available.

Assessing air quality alongside the SRN 

This section contains links to five separate commissions where we've reviewed specific sections of our network in greater depth.  

The UK Government published a plan to improve air quality across the country, including motorways and trunk roads. National Highways was asked to undertake detailed assessment of certain sections of the network where air pollution was thought to be high and there was a risk that members of the public could be affected. This wasn’t all of the network; the list initially identified a number of links based on national air quality modelling.  

This review has involved a number of different stages including an initial data gathering exercise of air quality in the local area, air quality monitoring and air quality modelling. The outcomes of this work helped National Highways to focus on areas above air quality thresholds.  

Each review is referred to as a commission. Over time additional links have been identified, with a new commission being requested for each batch of new links.  

This section of the library contains all the reports of the work completed to date.  

Commission 1

Commission 2

Commission 3

Commission 4

Commission 5

Monitoring 

This section contains information about our monitoring network, including automatic air quality monitoring and diffusion tube monitoring.  

Automatic monitoring 

We've been monitoring using automatic analysers alongside our network since 2016. This is when the National Air Quality Monitoring Network (NAQMN) was created.  

The following reports provide a summary of results from the NAQMN. Data from these monitors helps us understand nitrogen dioxide (NO₂) and, more recently, particulate matter (PM) concentrations around our network. 

This data is used to inform our programme of activity to improve air quality. 

Diffusion tubes

We've been monitoring using diffusion tubes alongside our network since 2022. These tubes are used as part of our PCM Evaluation Strategy [link] to determine when links are compliant (e.g. below the NO₂ annual mean limit value) and can be retired from the programme.

All our annual diffusion tube data can be downloaded from National Highways Open Data and is presented on our National Highways Air Quality Mapping. Our diffusion tube data is also provided in our Annual Evaluation Reports monitoring appendices. 

Research 

This section contains more information about different ideas we've considered to improve air quality alongside our network, and trials we've put in place to test these ideas in real-life conditions.  

Research compendium 

We've led a wide-ranging research programme exploring all potential opportunities to address poor air quality alongside our network. We've considered expertise from across the UK and internationally, and a comprehensive summary of our findings is provided here.  

Research compendium 

Since the publication of the research compendium we've continued to progress with our programme of air quality research. 

Speed limit trials

We've trialled 60mph speed limits on five short sections of our network where actions needed to be implemented to improve air quality. We've completed these trials and published the results of this research.  

Light vehicle motorway nitrogen oxide exhaust emissions

M1 speed limit trial analysis report.

Further analysis of the impact of speed limits on roadside nitrogen dioxide concentrations report.

Environmentally optimal tyres

The project was to determine the optimal environmental characteristics of tyres and brakes, in terms of noise emissions, particulate emissions to air and water, and rolling resistance to gain a more comprehensive understanding of the environmental impact associated with the manufacturing, use and disposal of tyres and brakes.

The report covers Phase 1 of the study and aims to determine the optimal environmental characteristics of tyres and brakes. It also considers how vehicle type and driver choices influence these resulting emissions. The analysis is based on the findings from the literature, engagement with stakeholders, market research and an evaluation of past consumer engagement.

Environmentally optimal tyres research

Electric Towns and Cities Initiative (ETCI)

A scheme offering grants towards the cost of electric vans for users of the A3 through Guildford was launched in September 2023. The scheme was delivered by Surrey County Council and Guildford Borough Council for National Highways as part of the ETCI.

The scheme was funded by National Highways from its Designated Funds. The aim of the scheme was to encourage businesses regularly using the A3 through Guildford to switch from their existing diesel van to an electric van. However, nearly 12 months into the scheme uptake was lower than expected and it closed in September 2024.

SmogStop barrier

We implemented a trial of the SmogStop barrier technology on the M1 motorway to determine the effectiveness of the three-metre high SmogStop barrier at reducing NO₂ pollution levels downwind of an active motorway.

SmogStop barrier

Microplastics

The environmental abundance of microplastics is a growing area of concern with an increasing body of evidence to suggest that the road network is a potentially important source area to surrounding aquatic environments.

The key objectives of this project comprised:

• a review of previous research
• establishing the scale of microplastics presence in highway runoff
• investigating the influence of factors on microplastics in highway runoff
• identifying and contextualising contaminants including new contaminants of concern
• exploring the benefits and options of reviewing and supplementing the dataset that underpins design and assessment tools such as Highways England Water Risk Assessment Tool (HEWRAT)
• recommendations

Microplastics

Elevated roads

This project successfully implemented a new method for quantifying the impact of the dispersion of emissions from elevated sections of roads within the widely-used ADMS-roads software tool. Improved modelling methods enable more accurate assessment of air quality impacts when elevated roads are to be modified or added to the SRN.

The model was specifically designed to calculate pollutant impacts in the vicinity of flyover-type elevated roads, but it's also useful for other complex road layouts involving elevated sections, assisting in making informed decisions regarding road layout and design.

Elevated roads