Reducing motorway speed may improve air quality – but more real-world studies are needed

While reducing motorway speed limits has potential to improve air quality, further studies are needed to understand how drivers respond to these schemes, according to a policy report released on UK Clean Air Day by the University of Birmingham.

Researchers in the Natural Environment Research Council (NERC)-funded TRANSITION Clean Air Network and West Midlands Air Quality Improvement Programme (WM-Air) reviewed research evidence on the relationship between traffic speed, air quality and health, including computer modelling and ‘real-world’ studies. Speed limit reduction schemes are of increasing interest in the UK as a measure to reduce air pollutant emissions from road transport – including nitrogen dioxide (NO₂) and particulate matter (PM) – with those living near major roads at increased risk of air pollution-related health harms.

Researchers identified that while speed limit lowering schemes introduced across the UK strategic road network (managed by National Highways) can improve air quality, the relationship between speed, air quality and health is complex with many factors to consider including the contribution of both exhaust and non-exhaust sources, such as brakes, tyre and road wear. Importantly, air pollution levels are also influenced by emissions from industry and agriculture, alongside seasonal and weather effects.

Modelling studies show promise that lowering the speed limit on motorways could improve air quality. These results have been supported by real-world speed limit trials in Wales which found that lowering the speed limit to 50 mph led to improvements in NO₂ levels. However, we need more information on health benefits from ‘real-world’ studies – including whether reduced speed limits encourage more people to take public transport rather than opting to travel by car.

The report also features computer modelling undertaken by WM-Air researchers to assess the impact of an average speed reduction from 70 mph to 60 mph on the M6 (between junctions 6 and 7) and M5 (between junctions 1 and 2) motorways. National Highways speed limit trials have been in place on these motorway sections since October 2020. Local air quality dispersion modelling results show that reducing average travel speed by 10 mph to 60 mph could reduce roadside NO₂ concentrations by an estimated 7-12% and fine particulate matter (PM_2.5) levels by 0.7-1% (compared with 2019 levels), with benefits up to one kilometre away.

While computer modelling studies typically assume all drivers will comply with regulatory speed limits, Department for Transport data suggests that under free flowing traffic conditions, 48% of drivers exceed the speed limit on motorways. Motorists may also change their acceleration or braking patterns in response to lower speed limits, or they may opt to take alternative routes.

Thousands of drivers around the world are voluntarily sharing their location data with insurance companies via black boxes or their mobile phones. This on-board GPS vehicle monitoring information, or telematics data, could play a leading role in strengthening our understanding of how changes in speed limits affect both driver behaviour and traffic conditions. Recently, we have been developing new approaches to extract valuable traffic and driver behaviour information from anonymous telematics data. This knowledge could also be used to guide future speed limit interventions - for example, those targeted at specific times or locations.

WM-Air researchers have used West Midlands telematics data to identify higher NOx emissions occurring outside rush hour periods due to higher travel speeds and accelerations. Such data could provide information to drivers to support vehicle speeds appropriate to local conditions.

With major pressure to advance progress towards net zero road transport and improve air quality, report authors highlight the need to optimise health benefits. Reducing vehicle speeds also has potential to improve road safety and reduce noise pollution, and future evaluation of speed limit schemes should also consider these broader public health and societal benefits.

Dr Suzanne Bartington and Dr Stephanie Lacey, Institute of Applied Health Research at the University of Birmingham