Air quality speed limit trials

What is happening?

We are trialling 60mph speed limits on short sections of our network where action needs to be taken to reduce emissions and improve air quality.

Based on the findings for our research programme we expect there will be a reduction in nitrogen dioxide (NO₂) when traffic is reduced from 70 to 60mph in these locations.

We are now trialling this approach on-road, to assess whether reducing the speed limit reduces NO₂ levels.

We will monitor this trial and if proven successful, the speed limit will remain in place until the area is compliant.

How do these trials work?

The 60mph speed limits are clearly displayed on roadside signs.

The speed limits are usually in force 24 hours a day, subject to operational issues. These include roadworks or breakdowns, which could mean we have to vary the times the 60mph speed limits for air quality are live.

Because we measure emissions on annual mean figures, the restrictions are in place around the clock to bring down emissions levels at these locations.

Why are we reducing speed limits?

We’ve been investigating the effects that different speeds and driving styles have on vehicle emissions.

Our research has shown that lowering the speed limit to 60mph from 70mph reduces emissions, which should improve local air quality.

Impact of coronavirus

Covid 19 lockdowns in 2020 and 2021 and reduced traffic flows impacted:

• traffic and emissions levels
• our ability to collect monitoring data.

This led to some pauses with the trials.

We reintroduced the 60mph speed limit over summer 2021 following the removal of lockdown restrictions.

Where are these limits in place?

We have speed limit trials at these locations:

• M5 junction 1-2 Oldbury - view consultation response
• M4 Harlington junctions 3 to 4 west bound

Speed limit trials at three locations are now complete, after evidence showed that air quality at these locations has improved:

• M1 junctions 34 to 33 Rotherham
• M6 junctions 6 to 7 Witton
• M602 junctions 1 to 3 Eccles

Three locations with poor air quality already had reduced speed limits in place, so we did not need to introduce new speed restrictions:

• A1 Blaydon Gateshead – permanent 60mph speed limit
• M621 junctions 6 to 7 Leeds – 40mph speed limit for safety during roadworks
• M32 junctions 1 to 3 – 60mph speed limit for road safety

Why these locations? 

The locations above were identified as locations where NO₂ levels exceed the legal annual mean limit level (40 µg/m³).

We have a duty to bring these locations into compliance with the limit value in the shortest possible time where feasible options exist.

Are these permanent measures?

We'll keep 60mph speed limits introduced specifically for air quality until monitoring shows that we can remove them. At this point, we'll reinstate the national speed limit.

Where we've introduced speed limits for other reasons, like road safety, they may remain in place after air quality improves.

If you have the evidence to show this works, why are these locations only trials?

Our research shows that reducing speed limits should should cut the time for trial locations to become legally compliant by 1 to 2 years.

We will monitor the trials to assess the impact in real world conditions.

If they're not having the desired impact after 12 to 15 months, we’ll remove them and explore other ways to improve air quality.

What will be the impact on journey times?

Reduced speed limits will have negligible impact on journeys.

The lengths of road covered by the limit will be up to 4.5 miles. The impact on journey times from 70mph to 60 will be minimal.

The lower speed limit will not affect HGVs as their legal maximum speed is below 60mph.

Evaluating the trials

We are currently evaluating:

• whether roadside monitoring shows a reduction in vehicle emissions
• how well road users comply with the signed speed limit

Assessing the impact of our speed limits for air quality trials is a complex piece of analysis and needs to be as robust and reliable as possible and this work is ongoing.

While we had intended to publish our assessment in spring 2023, the complexity and scale of the data (over 1 billion individual data points) means that we have not been able to and we are working hard to complete this work. We intend to keep the trials in place while we complete our analysis. This is to ensure that any benefits to local air quality are not lost in the meantime.

Why a 24-hour speed limit?

We need to improve air quality at these locations as quickly as possible.

Emissions levels are calculated on an annual average basis. Having speed limits in place 24 hours a day will bring down the annual averages in the shortest time.

Speed limits enforcement

Any speed limit displayed in a red roundel is an enforceable, legal limit. It is a driver's responsibility to obey the speed limit.

Enforcement is a matter for the police. The penalty is the same as for breaking any other speed limit.

How do you monitor air quality?

We monitor NO₂ using diffusion tubes. This is the recognised approach set out by Defra, which is also used by local authorities.

Diffusion tubes are small plastic tubes containing a chemical that absorbs NO₂. They allow us to measure the amount of NO₂ in the air during the monitoring period.

This is a cost effective way for us to measure NO₂ levels across many sites, both on and off our network.

We also have a system of approximately 60 automatic air quality stations, measuring NO₂ across our motorway network.

This gives us data on existing air quality close to our motorways and helps us evaluate the impact of new technologies.

Why 60mph?

We’ve investigated the effects of different speeds and driving styles on vehicle emissions across a range of vehicle types.

Our initial findings show that managing speed at 60mph has the most significant impact, reducing emissions on average by 17%.

This potentially brings air quality compliance forward by 1 to 2 years.

This is why we are now implementing speed limit trials on our  network.

You can read full details in this research report.

Do you plan to roll out trials more widely across the network? 

We have constant engagement with government about:

• locations on our road network that have poor air quality
• the means by which we can improve them and the time it will take us

Why should I travel at 60mph if I am using an electric car with no exhaust emissions?

To be effective, speed limits must be plain and clear to understand by all road users.

For practical and safety reasons, it's not possible to have vehicles of the same class (that is, cars) following different speed limits on the same stretch of road. It doesn't matter what powers the cars.

This is especially so on the short stretches of road where we're trialling speed limits for air quality.