Do traffic management measures such as 20mph and speed bumps increase air pollution?

Traffic management plays an important role in controlling speeds and we want to create safer environments for everyone to move in. We’re continuing to measure the impacts of 20mph speed limits and through our Congestion Task Group we will look at any ways we can smooth out traffic flows and that might mean looking at particular junctions or areas we can improve.
There are many factors which impact upon air pollution levels on a daily and annual basis and it is difficult to quickly and easily separate the influence of these from the impact of measures such as the introduction of 20mph limits. How drivers respond to 20mph limits, whether they are enforced effectively, whether the number of vehicle trips changes and the driving styles on a particular road are also likely to be important in the overall air pollution impact of this measure.

A number of studies have been conducted into the effects that 20mph limits have on air pollution. Overall, a smoothing of driving style and cutting out of the acceleration phase from 20mph to 30mph is considered to be beneficial for emissions of harmful pollutants. A study from Berlin concluded that the introduction of 30kph limits (18.6mph) resulted in up to 30% reduction in particulate emissions and 15% reduction in traffic related NOx emissions but enforcement of the limit and smoothing of flow with traffic light coordination was thought to play an important role. A 2013 study conducted in London, backed up with real-world monitoring showed the impacts of 20mph speed limits to be mixed but with particular air quality benefits being seen for particulate and NOx emissions from diesel vehicles.

Overall, the evidence suggests that 20mph limits are likely to result in negligible impacts upon air pollution. The Draft NICE Guidance on outdoor air quality suggested that 20mph limits should be considered as an air quality improvement measure but the evidence supporting it was found to be weak. Evidence of a worsening of air pollution from this measure appears to be limited, with a number of studies highlighting potential air pollution improvements in certain circumstances. Wider benefits from 20mph limits include increases in walking and cycling, reductions in noise, and a range of co-benefits including reductions in casualties and severity of those which do occur as well as proportionately larger positive impacts for poorer communities which otherwise suffer from higher levels of road traffic pollution. Therefore it is considered that 20mph limits should be seen a positive measure for both safety, encouragement of modal shift and air pollution.