The intention of this would be to reduce nitrogen dioxide (NO2) pollution, which, along with nitrogen oxide (NO), makes up the wider banner of NOx (nitrogen oxides).
NOx can have adverse effects on health, particularly among people with respiratory illnesses such as asthma. The Royal College of Physicians says air pollution is responsible for around 40,000 premature deaths every year.
NOx also contributes to smog formation and acid rain, damages vegetation and contributes to ground-level ozone. The UK is falling increasingly behind its targets, with 37 out of 43 regions currently in breach of limits.
In the Government’s report, it says there is “considerable uncertainty, however, on the real-world impact of speed limits on NO2 concentrations” and that “there is a need to collect data from further monitoring in real-world conditions - for example, at sites where variable speed limits are used already for traffic management purposes, to understand better the likely impact that different speed limits might have on air quality in differing circumstances”.
There is, however, some previous research, so let’s evaluate.
In 2011, the European Environment Agency (EEA) published a report on the subject.
This study documented a detailed simulation into the effects of dropping motorway speed limits from 120 to 110km/h (74.5 to 68.4mph) for Euro 4-compliant petrol and diesel cars with engines between 1.4 and 2.0 litres in capacity.
With smooth driving and complete compliance with speed limits, the diesel car exhibited a 12% reduction in fuel use and the petrol car an 18% reduction. This is because there is less wind resistance, so the car needs less relative power to propel itself. However, in more realistic driving, including some periods of speeding, this came down to a mere 2% and 3% respectively.
There was also a reduction in nearly all pollutant emissions, and especially NOx and particulate matter (PM) for the diesel – NOx by more than 20%, PM by around 10%.